earlwb

Sorry about that, I forgot that they miht have two raised center bosses on some of the engines. Yeah you have to carefully file them down a little.
But yeah the mufflers just bolt right on with little or no effort.

The original mufflers had a flange that wrapped around the engine exhaust outlet. There was also the earlier engines that had a rotating exhaust baffle coupled to the carburetor. The first type had no flange and used the center holes to mount the muffler and allowed the exhaust baffle to stay in place. Later they used the center two holes for those engines and a flange wrapped around covering the holes that was left over after you removed the baffle. Then they had just the flange and the two center holes. Later they made the engines with the outer two mounting tabs and no exhaust baffle and you could use either the center or the outer tabs to mount the mufflers.

The .29's were made almost from the beginning of the first stunt .35 engines back circa 1948 or so. The RC versions came out in the 1970's. They still made and sold the engines up until the early 1980's. Nowadays only the stunt .35 survives in a lapped piston and a ceramic cylinder version.

The .29X RC with a guillotine exhaust baffle couple to the throttle first appeared in 1970. The .29 RC with the rotating exhaust baffle coupled to the throttle and two screw holes in the center for the muffler first came out in 1972. If I remember right around 75 or so, Fox came out with the side mounting tabs and dispensed with the exhaust baffle. The Fox .36RC came out then and they still had some .29RC versions too. I forget when but Fox started bead blasting the crankcases to give them the dull gray color look instead of the shiny polished aluminum color look.