Checked my mailbox just now, I got a small package with thick supposedly 40A O-rings ("rubber gasket") for Cherry MX keys.
They look alright so they are probably fine O-rings. Can't say for sure, though, since I haven't actually tested them out just yet. Perhaps I will report back on the status of the Blue O-rings at a later date. I already put slightly thinner Red O-rings on this keyboard and they seem to work fine since this keyboards "Content" switches seem to be slightly shorter than the Cherry MX switches.