Bedbugbilly said
"...so what was the purpose of the "pinch"? Was this to give a "tighter" sight picture - i.e. as you aimed, it centered the front sight quicker/easier."
Someone will weigh in with a more expert response, but your guess is likely as good as any explanation. I have heard similar: that the rationale was that it limited any chance of eye to rear sight to front sight alignment variation beyond "straight on" sighting, I.e., that it "had" to hit POA...what you saw is what you got (theoretically). In reality, it makes it very difficult to quickly focus and therefore wasn't--and isn't--considered a functional plus. Accordingly, it has not been repeated in any production numbers since. In fact, many shooters open up the "standard" non-pinched sight channel more especially on Colt clones.