The Square/C barrel mark became the proof mark as well as identifying the barrel as being manufactured by Colt. This is a post WWII barrel with the Square/C proof mark, with COLT .45 AUTO marking on the top of the chamber. We know the hammers were not proofed, so maybe it was a case of not throwing anything away that might be used again, and the Square/C was later used to mark contract parts.

It appears this one was struck more than once, and with two different size dies.