I think a knock-off switch is a switch in the machine that will remove credits (or games) the player has won and built up on the machine. The player would have the owner or attendant in the location where the machine was located to come over, see how many credits/games you had on the machine, pay you for them and then he'd press the knock-off switch to delete the credits/games. This was a popular setup back in the old days on pinball machines that were being used for gambling reasons. The knock-off switch was inside the machine, so from the outside you couldn't tell it was rigged that way, the machine looked like a normal "for amusement only" device. When this was done there wasn't a hopper in the machine, if there was a hopper that would make it a gambling device, not an amusement device.
I don't know about the bill acceptor plug, maybe dormi or someone else will comment about that.