Miriam, I can find a lot of info on the featherweight blackside and they talk about how there were only two production runs of them. Of course, the aluminum would make a difference since it'd be used in planes during the war.
I can't find people talking much about a 201 blackside or even the 15. So I don't know if they're more or less rare than the featherweight blacksides.
I sent singer a question. I'm not sure if they'll have any questions but it doesn't hurt to try.