Can't say for sure when they started, Joe. Brophy says the Hunter Arms proofmark first appeared about 1922. Walt Snyder can perhaps contribute further on Ithaca, but his book has proofmarks (of the individual who proofed the barrels) starting in 1930. Proof testing was in force at both Winchester and Parker by 1930; not sure how much earlier. Turn of the century, back when there were a lot of Damascus-barreled guns . . . not sure.