From the introduction of what Parker Bros. called Vulcan and Titanic Steel barrels in the later 1890s we see two barrel steel markings near the front of the right barrel flat. On the guns marked Titanic Steel on the rib we see a letter
K --
131783 03 K in barrel Steel Location.jpg
I've also recorded the
K on a few barrels marked Acme Steel in the rib.
On the guns marked Vulcan Steel on the rib we see a
Kf --
118801 02.jpg
Circa 1906, in the 135xxx serial number range we begin seeing the V, P, PS, T or A in a circle barrel steel markings.