If we are to learn about Ven.Nanavira Thera’s correspondences, we would come across many subject matters the Thera had been acquainted during the time of a University student , that he makes use to clear concepts that is connected to explanations of Dhamma matters clearing doubts of the subject matter.

For example we can consider his correspondence with Mr. Wijerama, in Letter No.6, dated 4th March 1964.

Nanavira Thera had made use of his knowledge on Mathematics, Science ,Philosophy and his knowledge on Dhamma to clearly distinguish continuous change or Flux that had been forwarded to support “this object is not the same object” concept by Mr. Wijerama

This is what the Thera responded to Mr. Wijerama’s idea.

Now for flux. I see that you make a certain distinction between physical objects and mental states: let us therefore consider first physical objects.
You say 'The idea of continuous change or that everything is continuously changing seems to me to be correct. But the difficulty arises when the idea is extended and it is stated that this object is not the same object. The chair that is in front of me being of matter is undergoing change. In that sense it will not be the same chair.
But in another sense but much more real is the idea that the chair is there and till it breaks down it will be so. This is still valid in spite of the changes that are taking place which may or may not be perceptible so long as the chair could be used as a chair'

The Thera citing more examples of a Mango Tree, A Blue Curtain, a Leaf of a Tree and a Spoon, explains

where the change takes place within one and the same sensible quality or characteristic, we notice that it is always the more general feature that remains invariable while the subordinate or more particular feature varies. This suggests that there may be a certain structure of change that must be taken into account whenever we consider the question of change; and if this is so, it will mean that the statement 'everything is changing' needs strict qualification.

If it is possible, in any given change, to make a clear-cut distinction between those features that do not vary and those that do, it will follow that the distinction between sameness and difference is absolute: in other words, that we cannot say 'approximately the same' or 'approximately different'.
(So long as we use the word 'approximate' at all that will be an indication that we have failed to make the distinction properly clear-cut, since 'approximately the same' means 'the same but with a difference' and 'approximately different'—i.e. 'somewhat different' or 'rather different'—means 'different but partly the same'.)
If this is so, it will follow that all change takes place discontinuously; for if 'same' means 'absolutely the same' and 'different' means 'absolutely different', there can be no intermediate category between sameness and difference.

Well, it seems Cakewalk for the Thera to define all that but for a lesser mind not familiar with all that reasoning, it might not be that easy to grasp.
Nanavira Thera also gets an opinion from a “Scientific source” on the matter.

The 'Gestalt' school of psychology has specialized in experimental investigation of perception of change, and has reported that every change that we perceive takes place suddenly and absolutely. Whenever a perceived change is described as 'taking place continuously' it is to be presumed either that the necessary analysis of a complex experience is beyond the power of the perceiver, or else that, unwittingly, rationalization has taken place.
(That we do, in fact, have experience of movement and other such changes is, of course, not to be denied; but these experiences are notoriously difficult to describe, and the problem of motion has puzzled philosophers from time immemorial.)

Ultimately citing more explanations Nanavira Thera dislodges the idea of continuous change.

It can be shown by argument that the notion of continuous change is self-contradictory (in other words, that it contains a short circuit somewhere).

The following almost sums up Nanavira Thera’s explanation to Mr. Wijerama.

Perhaps you will be wondering why it is that I am so anxious to destroy the notion of flux—or at least to eliminate it from the context of the Dhamma (I have nothing to say against its use in the context of science, nor have I anything to say against science itself in its proper place; but its proper place is not the Dhamma: scientific thinking and Dhamma thinking belong to two quite different orders, as I hope to have made plain in the Preface to the Notes). The reason is to be found in your letter itself. You say 'The word flux means continuous change. If this idea is applied to everything it would be correct to say that what I see now, e.g. a tree, is not the same as I continue to watch it as it is subject to continuous change' and also 'I have heard as an extension of the same idea, Buddhist monks saying, pointing to an object, that the object is not there'. This doctrine is a complete misunderstanding and is wholly misleading. And, as you quite rightly point out, it is based on the notion of universal flux. In order, therefore, to undermine this false doctrine, it is necessary to point out that the notion of flux, at least as applied to experience, is a self-contradiction.

The Buddha has said that 'What is impermanent, that is suffering; what is suffering, that is not-self'.
This is understood (or rather, misunderstood) in the following way.
Impermanence is taken to mean continuous change (flux), and (as you have said) if this notion is correct, the idea of a thing's continuing self-identity cannot be maintained—what appears to be the self-same tree persisting in time is not really the same since it is continuously changing. In consequence of this, the idea of self is an illusion; and it only persists on account of our avijja, or ignorance of the truth of universal flux. If we remove this ignorance, we shall see that what we formerly took to be a lasting (or existing) selfsame tree ('A = A', the Principle of Self-identity) really has no abiding self at all—it does not really exist. And this explains why 'what is impermanent, that is not-self'. And what is wrong with this? What is wrong with it is—as perhaps you have noticed—that it does not explain why what is impermanent is suffering, and what is suffering is not self.

The above passage seems to contain lot of information that would be easier to understand if sentenced separately as below.

Impermanence is taken to mean continuous change (flux), and (as you have said) if this notion is correct, the idea of a thing's continuing self-identity cannot be maintained

what appears to be the self-same tree persisting in time is not really the same since it is continuously changing.

In consequence of this, the idea of self is an illusion;

and it only persists on account of our avijja, or ignorance of the truth of universal flux.

If we remove this ignorance, we shall see that what we formerly took to be a lasting (or existing) selfsame tree ('A = A', the Principle of Self-identity) really has no abiding self at all - it does not really exist.

The above tallies to the idea Mr. Wijerama put forward to Nanavira Thera. Let us see what the Thera says now.

And what is wrong with this?

What is wrong with it is—as perhaps you have noticed—that it does not explain why what is impermanent is suffering, and what is suffering is not self.

So we can see that the gradual change or the universal flux does not explain suffering due to impermanence and suffering that is not self. The Thera stresses out that

Suffering (dukkha) is the key to the whole of the Buddha's Teaching,

'Both formerly, monks, and now, it is just suffering that I make known and the cessation of suffering.' <M. 22: i,140>

and any interpretation that leaves suffering out of account (or adds it, perhaps, only as an afterthought) is at once suspect. The point is, that suffering has nothing to do with a tree's self-identity (or supposed lack of self-identity): what it does have to do with is my 'self' as subject (I, ego), which is quite another matter.
As I point out with the question of a thing's self-identity (which presents no difficulty) the Buddha's Teaching of anatta has nothing whatsoever to do:

anatta is purely concerned with "self" as subject'.

But this is very much more difficult to grasp than the misinterpretation based on the notion of flux,

so flux inevitably gets the popular vote (like the doctrine of paramattha sacca, of which it is really a part).

This is just a sample of correspondence the Ven.Nanavira Thera had with the few individuals who were horning up their Dhamma skills with Nanavira Thera. Sections of the Letter No.6 appears in Times New Roman fonts in white background. It is advisable that one should read the complete Letter No.6 , since this article contains only parts of the Letter No.6.

The “Letters” contain a wealth of information that is not to be found in anywhere else. Hope this had been useful to understand the ability and the Dhamma knowledge of the Ven.Nanavira Thera which in turn would help us to grasp Dhamma matters properly.