It's the arched mainspring housing.
Original 1911s did not have it. In 1927, they made some recommendations to the design that were intended to improve performance by the average soldier. These included, a divot cut into the frame behind the trigger, a shorter trigger, and the bump on the mainspring housing. (I believe that the divot cut is the only way to tell a 1911A1 frame from a regular frame.) The intent was to raise the point of impact. They determined that most people who missed with the 1911 were missing low. They figured if they put that bump in there, it would force the aim higher. It's a part you can replace. If you order the (free) Brownell's 1911 catalog, you will see options for arched, flat, plastic, metal, checkered, with lanyard loop, or without. If you don't like the one you have, you can switch it out.
Notice the gun you show on the left has the arched MSH, and divot behind the trigger, it's made to represent a WWII era 1911Ai. This is how the military kept them until they were retired in 1985. (ish. My arms room still had them in 1993.)
I actually prefer the bump, but most guns made today don't have them. I haven't gotten around to switching it out yet.