Quoted from pinballdoc:The research we did, was we went to the AFG registry owners list, and looked at the serial number of 979, which was only off a few #'s from 980.
Unfortunately there is no real correlation between serial number and plaque number, as the serial numbers are not sequential. The serial numbers are all over the place and I don't even think you could even get it right within 100 plaque numbers. Without the plaque there is no way to know what the real number was.
I don't believe you did anything malicious. It was an honest attempt to replace a missing part, unfortunately, it was a very important part of which was obviously incorrect. I didn't get to see the original eBay auction. But, I don't think "guessing" at a number is fair either. Maybe just sell it without the plaque or in BOLD letters describe that it is a replacement plaque that may or may not be the correct number.
Mine is missing the certificate #0017. I had a reproduction made, but I would definitely disclose that fact if I ever decide to sell.