I spoke with Benelli customer service today and they suggested to check with Numrich (which I did, same o-ring, over $9) or authorized dealers. I'll ask my gunsmith about it and report back. I seriously doubt that the o-ring is made of anything special but I could be wrong.
Oh, I also requested and received an electronic document from Benelli that is a detailed schematic of the R1 with all parts and part numbers listed and illustrated. If anyone wants a copy, PM me with your email address and I will send it to you. The document is entitled "ARGO Spare Parts List" if you request it from Benelli.
Regards