If you are looking for a non-correct aperture, it isn't going to be a true exact retro anyway.
The aperture won't even be noticeable to anyone but you.
I have an A1 aperture, it is small enough that I don't see how much smaller a 'NM' aperture could be.
It would be a simple matter for a qualified machinist to drill and tap an A1 aperture, and make some inserts that you could thread in, of various aperture sizes.
But that isn't going to be 'retro' so I don't know what you expect to find.