The one on mine just shredded off while removing the Envis lens assembly last night. I ordered already two replacements online (ORingUSA seems to carry mil-spec o-rings).
Question: since I expect to frequently exchange the Envis assembly with the C-adapter, is it worth it to put it back? I do not expect use under rain, splashes, etc., and with all of my astro equipment after use I always leave the item in the open in a dry environment at least overnight, to dry any dew residue.
I was just thinking that those gaskets are not designed for frequent on-off use.