Recently I bought a set of kavan muffler and adapter (no.4) for my merco .61 mk2. I am pretty sure it is a mk2 as it is a twin plug version as described here (http://www.iroquois.free-online.co.uk/merco/61mark2.htm).
However, the included adapter doesn't fit the engine exhaust port nicely, it does fit the kavan muffler nicely though.