Just realized that one of the cut-down 6-pounder cartridge casings from my dad’s collection of relics from the 1926 explosion of the U.S. Naval Ammunition depot at Lake Demark, Dover NJ isn’t a Navy 6-Pounder at all. In fact it’s a 1918 Army casing from the first tank gun. Probably not too common either because the Army was apparently through with the Tank Gun Mk 1 well before the 1926 explosion. Too bad they didn’t do a better job of stamping “TANK GUN.”
I have had one of the 6 pdr cartridge cases with added “Tank Gun” marking, but it was still full length. This one seems to have been cut down to blank/saluting length, perhaps taking advantage of any cases on hand regardless of past usage/reuse.
You’re lucky you found a full-length one. I believe this one was cut down to make a base for a wooden training round… That was after of course when the Army was finished with the Hotchkiss rapid-fire 6 Pdr inside a tank. (The reason I say training round is because there’s a brass plug pressed or soldered into the hole for the primer.) Thanks.