I talked a friend into buying a1931 Detective Special. It's a very nice revolver but it has a bobbed hammer. My friend would like a full hammer. Will a "D" frame hammer work if we can't locate a "C" frame, more historically correct hammer?
There's no such thing as a Colt "C" frame revolver.
You can buy used hammers, but you need to try to get one of about the same general age as the gun, since there were some slight differences in the hammers.
As example, on the right lower side of the hammer is a clearance cut to clear the hammer blocking safety assembly.
The cut changed shape on newer guns and a newer hammer might not work.
You'll need a pre-war type hammer.
In the pre-war days, Colt didn't use letter codes for frames, so technically, that gun isn't a "D" frame, it was known as a "Police Positive Special frame".
Only after the war was the frame assigned the "D" nomenclature.
In any case, Colt hammers will likely need at least some hand fitting to operate properly, so I suggest getting a hammer installed by a Colt qualified gunsmith.
You can buy pre-war hammers at Gun Parts and Coltparts, but you need to talk to them to insure you get a pre-war hammer in good condition.
We're not 100% sure of the grip but I think the material is staghorn. Here's a shot of the bottom of the grip. There's a void in the original material. The color is similar throughout. I'll contact Gun Parts or Coltparts for the hammer.