Concur with comments, that is a really sharp looking half-A stunter you have there, Robert, and with the rarer OS (Pet?) .061 nonetheless.
(OT) I purchased an OS Pet .06 from the now defunct Hobby Shack, Fountain Valley, CA in 1973. It was for my 36" GB Jr. Falcon R/C. To my surprise, they substituted without cost an OS Max .10R/C (1965 version) with butterfly valve exhaust restrictor coupled to the R/C carb. I powered the Falcon with a Cox .049 Golden Bee instead, but put the Max .10R/C in a 39" Top Flite Schoolmaster all sheet. Flew it on Ace R/C Pulse Commander rudder only with KRD sequential quick blip throttle servo. Wow, what an engine, full throttle it climbed out at 45 degree angle, half throttle like a .049, and whisper quiet at idle for low passes. Yet, at full throttle, its noise was only that of an unmuffled Cox reed valve .049. It is such a great engine that I have 3 of them.
Sorry for departing off topic. I imagine that OS would also make a decent C/L engine as well, being as strong of a cross scavenged engine as it was. Need to get off my duff, but got an original 36" Sterling Profile P-38 and 2 Enya .09-III with venturi intakes reserved, and get it built.