Well I checked some of my Ryobi's and assorted clones. I have one 2 ringed Ryobi with slits. One single ring Ryan without slits. Also one of the Ryobi/Ryan motors that Cvan mentioned from a Stihl weedeater; 1 ring no slits.
Like Herb mentioned, It would be interesting to see how a 2 ringed piston would run in a cylinder with no slits.

I know a guy that has a lot of older Ryan weedeaters I may have to go visit him.
Adam