When Mamod stopped making the Meccano engine for Meccano (to be sold under the Meccano name) they renumbered it SP3, updating it with a solid fuel burner, water sight glass and whistle. Thay also changed the colour to metalic silver-grey.
The boiler is a fairly standard Mamod one. It has a chrome plated cover extending right over the top of the boiler, possibly at Meccano's insistance for safety reasons. The boiler is 1 3/4" dia x 4 1/2" long. It has a standard Mamod 1/4" safety valve, a water level sight glass and a whistle, but no steam dome.
The engine is a single cylinder, single acting oscilating cylinder type with a reversing lever which operates by shifting the trunnion. The crank shaft and flywheel are almost standard Mamod except that the shaft diameter is Meccano standard 5/32". The crankshaft was fitted with a Meccano standard gear wheel and chain sprocket, but they are missing from this example. There is a pulley turned into the boss of the flywheel.
The whole thing is riveted together, which would make restoration a bind. Fortunately, it is in reasonably good condition so I've left it as is.