I believe there were both threaded and state-compliant Mark III Lights. They look similar but the state-compliant ones only appear to have a thread protector nut and the barrel is actually not threaded underneath. The standard thread pitch on .22s in the US is 1/2-28 UNEF; to my knowledge all threaded Ruger .22s use this pitch.
"to my knowledge all threaded Ruger .22s use this pitch."
Generally true BUT Ruger ended up with a bunch of guns threaded for the "European standard" (1/2x20 ???)that places like CDNN sold with an adaptor to fit 1/2x28.
Do you have model numbers or links, Mobuck? I've seen several 1/2-20 threaded .22s on CDNN but never a Ruger. I was also unaware that Ruger exported enough .22s to the European market to make it worth their time to use a different thread spec.