Just an update... I found the washer. It was hiding in a crack in a baseboard.

Now I have everything on a tray in labelled ziplock bags, ready to reassemble. Taking a deep breath here...

Thanks very much for your help and the link was great. A lot of information.

Amazing how small parts find cracks to hide in.
When you get it back together you may want to build a speed tester described in this thread:
https://graflex.org/helpboard/viewtopic.php?t=6105
I have one with movable sensors so I can test multiple formats. It uses two sensors, one at the top right and one at the lower left but the curtain speed test the same with a single sensor at the center.