"There is no cylinder stop screw on the base of the trigger guard."

Although the old adage is that nothing is impossible with a S&W, that is about as close to being impossible as you can get with that SN. The first model change without the trigger guard cylinder stop screw was the 10-4 in 1962. That change would have occurred somewhere in the SN C5xxxxx range but I don't have the exact number. Are you looking at the right place - at the front of the trigger guard?