three digit Colt SP1

[Johnny in Texas]

My guess is the ATF required Colt to make the change. Your correct many firearms will slam fire with-out a sear. Since the advent of the AR series or rifles and the many semi auto sub-machinegun type pistols that have entered the market since the Vietnam war have changed the way sears can function. Like no open bolt semi auto pistols aka MAC 10 MAC 11 UZI's and a hoard of others.

[BigMo]

Johnny in Texas:
The changes you are thinking of were much later and a different model designation- According to "Black Rifle II" that was around 60,000 serial number but no recall of previous rifles, just a change to new production rifles. In the same book, the author writes it was done for safety reasons and not due to ATF pressure- that came later.