Hello Tony
A very warm welcome to You
Those rubber rings are from a very early clutch (XS2) Part No. 256-16367-00. Described as Cushion Ring in my parts manual. I was kind of surprised to see that u have them in a 2FO engine but with these bikes being 50 years old, i guess i should'nt be surprised at anything anymore.
As far as i can make out, they were an early attempt by Yamaha the resolve the dragging clutch issue and were discontinued after the XS2 I think. Lots of info in the Tech section about the many clutch changes, if u fancy a long read.
Many attempts have been made to improve these clutches by many members as you'll see in the Tech section but by and large they do work better with more modern friction material.
I would not use the rubber rings with new EBC plates as it should work perfectly well without them. The cushion Rings, if in good condition, actually put a small opening pressure on the plates to force them apart when it's disengaged and this may not always be desirable.
Oh and the same size friction plate was used on many Yamaha models right up to the first model R1.