For a while, this engine was unique. I mean that in it's literal sense, not just unusual. I know of a couple of other similar (but better) examples, built by my good friend, David (aka Stilldrillin'). David has also made a 4 cylinder version. If anybody knows of other examples I would be interested in hearing about them.
(See here for how it came about.) It was derived from a pair of SE3's (see here for a similar model) and all parts (apart from a few trivial bits like screws etc) are Mamod. The boiler is a standard SE3 boiler of unknown vintage (probably 1960's) but with the throttle removed and the whistle replaced by a pressure gauge. The latter came from a modern Mamod engine. The boiler's dimensions are 2" diam. x 6" long. Steam feed from the top of the boiler is taken down under the boiler and over the burner, to give a modicum of superheat before the long run to the cylinders. Two of the cylinders are from one vintage and the third from a different vintage, although all from SE3's. Their dimensions are 5/16" diam. x 3/8" stroke.
After having solved a few teething troubles, this engine steams and runs reasonably well. It exhibits all the attributes one would expect from such an engine, e.g. good torque and smooth running right down to quite low speeds (for its size). I'm glad I took the trouble to perform this experiment. Maybe the powers-that-be at Mamod, if they read this, might just take the hint.
In one of the photos on the left it can be compared with an example of the SE3 it was derived from.
Due to the extra length of steam pipe, it was sometimes prone to problems caused by water in the cylinders, especially the one furthest from the boiler. In order to reduce this, I have lagged the steam feed pipes with string, as can be seen in the last photo.