"Most", though not absolutely universal, guns found with the two piece top rib will be those having a top bolt or Doll's Head. The short rear section will be brazed along with the breech jointing & under lump, with the remainder of the rib being soft soldered. Those guns not using a rib extension will more often have a one piece top rib, which will be soft soldered in its entirety.

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