That's not a proof mark but a military acceptance stamp. Proof marks weren't introduced until 1892, so it's unclear if your rifle has any. I would take the barrel out of the stock and look underneath. I would also take off that scope mount on the front of the receiver, it covers the area where you would usually see markings.
I used to love being able to hit hard at 1000 yards. As I get older I find hitting a mini ram at 200 yards with the 22 oddly more satisfying.