The square faced trigger guard came about by demand for it created by some off the wall TV series in the early '70s that featured a Detective that was shown quite a few times with his support hand index finger curled around the front of his trigger guard. Competitive shooters and Cops started having a Pistolsmith metal checker the front of their rounded trigger guard so they could also shoot that way without their finger being in danger of slipping. Several years went by and the handgun manufacturers ever seeking to cater to demand, no mater if the demand was for a solution to a non existent problem or not. Handgun trainers dutifully tried to make that shooting technique work. By about 1980 it was all but universally discounted as being anything more than a TV 'for dramatics' technique and not near as good of a grip as what is in common use today. Manufacturing being what it is, the response time to eliminate the square front trigger guard took different times for different models.
As a Pistolsmith, I did a lot of metal checkering on the front of rounded trigger guards because that is what the customer wanted. Later, I did some reshaping of square front trigger guards eliminating the blocky look for other customers. As a Firearms Trainer, I tried the support hand index finger out on the front of a couple of flat front trigger guards and determined that it was not near as good of a grip as is placing that index finger directly under the trigger guard and pressing up while shooting. ...