From my observations the K
f was the barrel steel mark for the steel Parker Bros. marked as "Vulcan Steel" on the rib legend, from their introduction up into the 13xxxx serial number range when we begin seeing the V in a circle. The guns marked "Titanic Steel" and a very few marked "Acme Steel" on the rib legends had a K in the barrel steel location on the right barrel flat.
131783 03 K in barrel Steel Location.jpg