Update
Result posted for the record and info of any others who might search in future with a similar query.

Since found several different sites with online images of same model O.S. MAX 20, but of later era manufacture when it came supplied with muffler in box which illustrates the small volume OS-702 was its factory supplied and recommended muffler.

That the spring metal band fits, albeit requiring a longer bolt of the same diameter and thread than the one supplied with the same muffler accompanying the MAX 15, and that it fits the 20's exhaust aperture perfectly reaffirm this. I scavenged a bolt-through muffler screw from an OS 15 FPS as a substitute and its diameter, pitch and length allowed fitment of the muffler to the 20 like it was meant for it. The OS-703 (which I have) from an O.S. 35 is much larger in volume and doesn't fit also affirms the 702 as the muffler for this unit.

Bench running the engine with this muffler fitted confirms both work OK. And although dB weren't measured other than by Mk I Ear, standard issue, 2 thereof, with a noise level typical of that which would have been considered acceptable for a silenced engine in that era and as expected from the 702's minuscule volume on this capacity engine.