Here are my Mercos.

Top left is a near new Merco 49 Black Streak with the twin plug head (the front plug can't be seen because it's got a much shorter terminal then the rear plug). This is the later version and what's called a "heavy case" because of the massive rib running down from the exhaust port plus the way the front end appears as though it could have been machined for a removeable front end.

Top right is a brand new 61 Mark2 in what's called the "light case" which is the same as the original Mk1 but with revised internals and of course the twin plug head. The twin plug heads were only used to get a more reliable idle for RC, the original engines were mainly used for CL stunt so idling wan't much of a problem .

Bottom left is an early genuine 35 stunt only engine as can be seen by the cast venturi. This particular engine has done massive amounts of flying time (as you can tell from the colour of the head which should be orange) and helped me win a state championship 3 or 4 times.

Bottom right is a slightly later 35 which used an inlet bored for either a venturi or RC carb. One thing about these smaller Mercos is that you don't ever want to lose the prop nut. It's a Whitworth fine thread (1/4x26) and probably near impossible to find today.

For some info on the early history of Merco and quite a few engine tests go to
http://www.iroquois.free-online.co.uk/merco/index.htm