I recently sectioned a steel 105MM M115B1 shell case and noticed something I have never seen or heard of inside. Located inside on the very bottom, next to were the primer tube extends are three good sized letters engraved in the metal. They read, “PRM”. Anyone hear of this before? What does PRM stand for?
I took a photo which did not come out all that great but you can still vaguely see the PRM at the 12 O’Clock position.