Doug;
Do these guns have Chopper Lump bbls. It would appear the rib may be hollowed out for weight reduction & either has a plug at the breech end or perhaps a projection from the chopper lump (if used) for the rib to set on. I had not noticed it until the question about it being two pieced was asked, but that is an unusual solder line for a rib.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra