Chip, I have rebuilt one 235 and two 261's in my own trucks in the last 6 years.None of them had any sort of thrust washer between the oil pump and distributer when they were disassembled. I also had a few other 55-57 235's laying around here, no thrust washers on them either.On the 3 engine, I replaced all the oil pumps with the M-45 pump which had a Sealed Power name on the boxes, all bought from the local machine show at different times.No thrust washer on them either.All these engines are in trucks driven on the road and none of them have any problems like you are experiencing.I'm no expert here, maybe I have time bombs ticking away too.
None of my several old Chevy parts book show any sort of washer, but it may be captive like Tom Says? Anyway the GM part number for a 55-62 Chevy oil pump is 3815635
