[Researcher]

From my collection of barrel flat pictures the early fluid steel guns only sported two different barrel steel marks -- Kf on the flats of Vulcan Steel barrels --

118634 02 Kf barrel steel mark.jpg

and K on the flats of barrels rib marked both Titanic & Acme --

92809 03.jpg

In the mid 13xxxx range I see the transition to the letter(s) in a circle markings -- V, P, P.S., T or A.