Experience has shown that no marks on these SKS carbines are applied willy-nilly. Each mark has a purpose, and many have been deciphered that once had no known meaning or were once misidentified through internet lore.
An order went out to stamp this Circle-Star on the right buttstock, either at the factory or place of special issuance. Could be for a special force, unit, geographical location, political faction, etc. And while it is fun and maybe even proper to speculate about its application, it is important not to make any mythical declarations about its identification without verifiable proof of origin.
Right now it's a pretty cool thing. Hopefully more will show, and some solid information can be attached to its purpose.
Meanwhile, thanks for posting!