Mike's barrels demonstrate that with most doubles with a top rib extension (esp. if the top rib contributes to a bolt lock-up) the extension is a separate segment. AND the challenges of coloring steel.
The extension was usually indexed in some manner with a screw
L.C. Smith which was brazed
Parker GH with poorly matched D4. On refinishing the higher carbon screw stained much more darkly
Some extensions on damascus barrels were a non-twisted (layered) laminate, others solid steel; Parker Lifter Bernard barrel with steel extension which stained more darkly
The iron component of pattern welded rods was no doubt wrought iron. The steel was either Bessemer/Decarbonized (carbon .06 - .1%) or 1002 - 1005 low carbon low alloy steel.
I've done composition analysis on ONE segment of rib, a c. 1912 Lefever Arms Co. DS, and it was AISI 1011.
So coloring different composition steel was and is real challenge. Hopefully Mike will post the final results.