The seat stays on the 58 RSL are 12.7mm vs 15.875mm on the CR. The stays on the RSL are also seamless 6/4 ti vs. 3/2.5 seamless on the CR.
The weld seam that you see on the seat tube is the thicker pressed-in ti seat tube insert. We machine this to be thicker than the seat tube wall in order to strenghten the top tube/seat tube/seat stay & seat post junction. It also helps dissipate heat during the weld process, so we don't warp the very thin seat tube. We press the insert into the seat tube and then weld it in place. Your Rigor has a pressed & bonded Aluminum insert. If you remove the post on either and run your finger down inside about 3'' you can feel the transition between the two pieces.
Let me know if you have any other quesitons.