Just found this thread via Google; same deal with my recently acquired '85 Super Le Tour. Came with spacer on non-drive side only. Wondering/hoping adding one on drive side will help with an occasional chain skip issue (yes, I checked the chain).
Might there be another one out there someone could spare? BTW, I also found a part by Surly called "Monkey Nuts 2.0" that look very similar - $25/
pr.
Or these "Karate Monkey" spacers (also Surly) $8/
pr: