Huh, I had no idea that PA had such a law. Here in Massachusetts we have to report gun transactions to .gov, which amounts to a de facto registration, but there's no requirement to register guns that were owned prior to the reporting requirement. I'm a couple of weeks away from a long-anticipated relocation to New Hampshire, where there's none of that BS. Can't come soon enough.
Thanks for the compliment, but I'm really just a guy who happens to own "the book" - i.e., the Standard Catalog of Smith & Wesson, which is a great reference that you can pick up on Amazon for short money, considering how much information it has. I don't think I can do much more with the photos, but please post them anyway - I'd like to see the gun and I'm sure others would as well.
Before you spend the $50 on a factory letter from Mr. Jinks, which still may not solve the mystery for you, you might try posting over on the S&W Forum, where the real experts hang out. They very well might have some ideas that haven't occurred to me, or perhaps carguychris might stop by with the answer.