He could ask folks for a pic of their serial number before selling them one.
Unless you have one of these last 100 games, there's really no point in having the plaque. The plaque has the serial number on it and notes that it's one of the last 100 of 5100. I haven't checked, but I assume the serial number is also listed on the back of the cab and maybe inside the cab too. No serial number on your game? Then no plaque for you. Serial number under 5000? This plaque isn't for your game. It would look totally wrong.
This game, serial number 00003, is one of the last 100 SWE1 games built out of 5100 total.
That's not the exact text, but you get the idea.