Gear box thrust washer

When my 912uls gear box on a 500 hour engine on a Rans S7S was disassembled thrust washer 827992 was missing. Using a borescope the holes in the crankcase behind the gear box, the hole where the banjo fitting is located and the drain for the oil tank were examined and no sign of washer parts was found. Don't know where to look next. Could the thrust washer have been left out when the gear box was initially assembled?