Quoted from pinhead52:
Look at the serial number database and pick an appropriate number for the game, ie 1 more then the suggested range
OK. So it looks like the serial numbers run between 04000 and 05600 based on a run of 1600 games. So I will pick 05606 just to have something on the game. I also thought about putting something out of bounds so it doesn't look like it might be the last game off the production line. Something like A1126.
Funny when I explain things that are important to pinball collectors to my friends they look at me funny. This one I will not try to explain...