I Googled and found this, per Wiley Clapp, American Rifleman, June 7, 2010:

"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".

Oops, Ardeebee was quicker, plus his link has the photo