In the final years (mid-'60s) that Konig manufactured the two-cylinder FB engine, it was delivered with a tuned exhaust that was, IIRC, a two-into-one pair of elbows terminating in a simple shallow-angle megaphone, which was enclosed in a big muffling "can" with a short outlet pipe. The early rotary-valve fours later got a similar system.

If anyone has a dimensioned drawing of the various parts of the FB "can" exhaust, I'd like to see it.