The word "prawl" means a bump, knob or projecting corner on the frame of a revolver. As best as I can determine, the term prawl originated during the 19th century. This is when repeating handguns first became common.
You occasionally see the word used in describing something like a S&W Russian or Number Three New Model.
The prawl had a useful function in firing the gun. Held in a positive grasp, a prawl-equipped revolver wouldn't move around when the gun is fired. It also tended to position the gun firmly in the shooters grasp when he reached for the hammer with the thumb of the shooting hand.
Peacemaker Colts, Remington Frontiers and S&W Schofields didn't have prawls in their design. Carried over to modern guns, you could say that a Model 29 .44 Magnum has a prawl of sorts.
The revolver on the left is a S&W Russian with a prawl on the top rear of the frame. The other one is the S&W American without a prawl.