Originally Posted by z1r
Yes, it does have a prefix. There's another on GB without, that is also marked .22 Cal.

See what Ruger has to say about the OP's SN:

https://ruger.com/service/productHistory/RE-SSixMM.html

Yes, I think I misunderstood your post. I knew, and he was already told, this is common with the 3screw SS 22 magnums.