I'm restoring a classic Stern and I found the following card with the game's serial number in the cabinet. I can't find where it was originally located. I've looked though pics on IPDB and can't find where it goes. I'm pretty sure it doesn't go on the underside of the PF as none of the pics have it located there. It also doesn't look like it went in the backbox, none of the pics show it there. So that leaves the inside of the cabinet, and there aren't many pics on IPDB of the inside of classic Stern cabinets.
The card is 2-3/4" x 1-3/8"