'N' = 1951 - 1st year of production for that "BANTAM" frame piece is usually stated as 1952 in the catalogues - but yours, if clearly marked as 'N' would make it VERY early in production, late in 1951 or very early 1952 fiscal year - that would also fit with the 5 digit sequencial production number after the prefix - about the number of handguns that would have been made in that catagory in 1951. This is the first and only example that I've seen (of that model) with an 'N' prefix. Very nice find.
I just checked my inventory and I do NOT have a M633 example at this time - but I do have a couple early 632s (the blued variant).
Check yours for the "All-in-One" ejection system - it should have it.