[Contenderizer]

Thanks both of you ... and by the way, you are both correct. Here is another definition of that I have found since posting my question:

Hemispherical outgrowths of the receiver of a double gun that mate with the breech ends of the barrels. The term derives from the flanges (or fences) in this position on a muzzle loading gun that were designed to protect the eyes of the shooter from sparks and escaping gasses.