For those of you who have a similar problem, I thought that I would tell what I finally did with my Marlin 336A and the little bitty hammer. I found a Marlin 336 Hammer with Extension for sale, on the Auction Arms website. I bought it for $10. While it may not be "original" 1950, I really don't care too much, because I can now shoot the gun with a scope on it. It is probably from the 1970's. The spur of the hammer looks like a "T" in cross section, same as a late model Marlin. It fits and it works. Nothing else I tried was ever going to get me something that worked. I was lucky to have found it.
If you see a EARLY model Marlin 336 offered for sale, and it has been drilled and taped for scope, don't assume that it will be easy to find a hammer extension to fit it. You could have a lot of trouble. I did.
Teton Kid