Cessna 120-140 Association

I am rebuilding a master brake cylinder in my 1946 140. It appears to be Cleveland Master Cylinder with
Push Type with Reservoir with the cover held on with a snap ring. I can find all of the AN6227B O-rings on Spruce. However, the reservoir O-ring has no number associated with it and the "Cleveland" one on Spruce is listed for an incredible price of $21.95!

This is just a simple flat edged ring about 1-3/4 x 1/16 (part #17 on attached file).

Does anyone know where these can be purchased for a reasonable price?

So, square cross section? Can you get the exact inside ID? Drop it on something cone shaped and measure where it sits.

Yes, good point. Actually it should be exactly the same as the cover plate - will check tomorrow. Thanks