I really like mine too -
My unofficial research - these were made in 1931 -1932 and appear to be the first group of 15-91's made. It does not appear that they were a special run or anything like that - just the way Singer made the first 15-91's. Most of the runs were small - 5,000 machines each, however if you add them up, I'd say there were 40,000 -50,000 of them made. That is a very small number for Singers, but not are rare as many sellers want you to believe.
Right now I'm on the hunt for a 15-91 manual from 1932, to see if it's got pictures of the silver trimmed machine. I found one that had been sold - so I know they are out there for that year, but the listing didn't show any internal pages.