I sent the store a message, asking as politely as possible what the serial number was. They probably think I'm nuts, and I am. But in a good, numerically obsessed way.
Just thinking of what numbers it isn't, with those features from that early in '77. Not; 595, 611, 613, 617, 619, 621, 624, 635, 637, or 640. Quite a few gaps to fill. I guess there's always the possibility of a duplicate number too.