Vintage Post WWII U.S. Navy Gun Barrel Plug

This is one of those very old items that we have trouble finding information on how it was used. However, we were told that it is some kind of U.S. G.I. gun barrel/cannon plug. Measuring 6” in diameter and 3” deep. Embossed on the outside it reads MK 4 MOD 1 KF Co. 1948 with a small anchor. These bowl shaped plugs are made of a heavy duty bakelite and are brown in color. We are assuming that the anchor symbol would make it from the U.S. Navy, so we’re thinking maybe it was used to keep the saltwater out of the weapons barrel. Unused Condition. MADE IN THE U.S.A.