I have owned very many pre 64 Win 70's and handled that many more. We are talking in the hundreds. The number isn't coming off due to wear.
The ones that I have seen that did not have a number were obviously ground off and I have seen a few of those.
I will venture a guess that although obliterated, there is a trace of a partial number. That is how I should have phrased it originally. As pointed out it is very easy to remove with emory paper and leave no trace. It is very hard to replicate the original electro-pencil engraving.
