David:
I think that the rubber rings simply make gripping the adjuster easier. There are some spare part vendors on the CR list who can probably get you NOS replacements. You can also try a local hardware store, where you might find something "close enough".
Lastly, my Campy 2001 "differential" sidepulls also have the same rubber ring. I'll bet that the new part will fit your older brakes.
regards,
Andrew Gillis (cloudy in Long Beach, good wool jersey riding weather!)