I have a model 16-4, 6", blue, that I bought new, when these
were first available. It had a dull blue finish, and some obvious
machine marks on it's exterior. I sent it to the factory with my
complaint/observations noted in a letter. A month later, the
gun was sent back to me, every problem fixed. The polish and
blue are better than any production revolver. The entire gun
was re-done, even the trigger guard is like a mirror, inside and
out. It was sent back with a letter from the "custom shop", a
letter I can not find to prove the work was done by S&W. There
are no additional markings on the butt, crane,frame, to
signify that S&W did the work. So, do I have a problem when
trying to prove "provenance", or will the quality of work speak
for itself when it is to be sold?

Welcome.One of the experts will no doubt be along shortly,but within my limited knowledge,there was not a "custom shop" in that era,nor have I heard "custom shop" mentioned on this forum.Only later was a "Performance center" created.

Old Skool, I am in no way an expert on S&W. Though you can't prove it was refinshed by S&W, if you were to sell it, the quality of the finish woud bring top dollar, anyway. No need to even mention it was refinished. It was probably done by one of the "old school" gunsmiths that S&W used to employ. Bob