I have a rear rotor G21 .29 just like the one in your picture. It was supposedly built in 1972 as a control line speed engine but I bought it (never run!) to use in SAM B Glow. I sent it to Don Blackburn, the SAM engine guru for tuning and he said he'd never seen one just like it as it seems to be one of the earlier ABC engines...most were conventional iron sleeves at that time. Is yours ABC? The large intake means they need to be run on some sort of pressure. Mine uses a presure tap in the upper left backplate bolt hole that runs into the bypass.

It runs real well, in the 18,000's. They seem to be the engine of choice in B Glow and are much sought after on auction sites.