The SP4 was a development of the SE2A, the main differences being the base mounted chimney and more modern SP series styling. This particular example has a customised colour scheme. I assume it was supplied by Mamod in this livery to a retailer by special order (see below).
Other than the colour scheme, it's a bog standard SP4. The burner is a standard solid fuel tablet type. The boiler is 1 3/4" dia x 4 1/2" long, with an end cap on one end only, fitted with a water level sight glass. It has the standard 1/4" safety valve. The whistle is of the normal spring lever type.
The steam feed has no attempt at superheating and goes directly to the cylinder block. It has no lubricator or throttle. The single acting oscillating cylinder is 8mm dia x 20mm stroke. The crank shaft is fitted with a 64mm diameter flywheel. The exhaust is fed via two pipes, one for each direction of running, to the base of the chimney.
As acquired, the engine appeared to be unused and in very good condition. There was no sign of it ever having been fired. (I have fired it up since, however.) It came to me in a box with a £19.75 price label, which also says "NUNN'S DISS 2205 SPORTS & TOYS". The box illustration has the standard colour scheme but that's to be expected. The previous owner says he also had an SP2 in the same colour scheme, in a box with a similar label. My guess is that it's a special edition made for this specific retailer. If anybody can provide some detail of this engine's history, I'd be grateful.