Well Bob, apparently the misunderstanding is mine. Somehow, in all my years of research and study, this little detail escaped me. I was under the impression that the V-notch 'was' the pinched frame.
Either way, the point remains that the blackpowder frame with the setscrew basepin retention, will typically have the V-notch sights. Whereas crosspin guns will usually have the square notch sights. Which, unless I'm suffering from more misconceptions, was something that didn't come about until the 2nd generation SAA's.