I didn't read that he was offered a swap of parts. We might be jumping to conclusions on that point.
If you have any housing with a single dogleg hammer mark, then I'd stay with that hammer.
I read where Rock-Ola sent 10,000 hammers to NPM 9-13-1943
One thing did catch my eye, the milled rear sight. I don't know if I have ever seen what I thought was an original NPM with an adj rear sight.