Belongs to a friend who received it from an older friend whose family had it for 3 generations. He thought it was an old Henry but side gate makes that a no. Any clues? It's the engraved brass one.
Hot damn, I think that's it. There's a good chance he was confusing the Henry caliber with Henry rifle and was just assuming 45 Colt. I know he has bore gauges and it probably checked out at ~45 caliber.
I'm waiting to hear back from him, but I think the friend who gave it to him had confused the caliber with the make of rifle. Not a Henry rifle, but probably takes a 44 Henry cartridge. My friend did say it was supposed to be a round you could no longer find, which doesn't make sense if it was 45 Colt.