It is called a trigger shoe. It clamps onto the trigger with 2 small Allen set screws that take a .050 Allen wrench to tighten or remove. Used to be quite popular in the 1950s-60s as an alternative to installing a target trigger. The best ones were the "Flaig Ace". Unfortunately, when the screws are tightened enough to securely fasten the trigger shoe to the trigger the cupped ends of the screws leave two circles on the side of the trigger. The other often asked questions is "What are the two circles on the side of my trigger?"
