The ME3 was a replacement for the ME2 and was unusual for Mamod in that it used an engine unit made by SEL. The rest of the ME3 was similar to the ME2, which was, itself, similar to the later ME1 but with a non-inclined engine unit and no stern tube or propeller shaft.
The boiler is 1 3/4" dia x 4 1/2". It has a water level plug and a combined filler / safety valve / "dome". It is mounted in traditional manner over a typical Mamod "vapourising" burner. The steam pipe exits from the side of the "dome" and, without any superheating, directly to the engine unit.
The SEL engine unit is their model 1560 "steam launch unit". (see here). It has a 7/16"diameter cylinder with a 1/2" stroke. The flywheel is only 15/16" diameter but is still quite heavy being 5/8" thick solid brass. The whole is mounted in a substantial solid brass frame and is not inclined. The output shaft is fitted with a pully, which has a pin at the rear end to form one half of a simple universal joint. There is no propeller shaft fitted.
As many of the steel parts were beginning to rust, I completely stripped down and repainted them all. The engine has obviously been fitted into a boat in the past as the base plate has been trimmed at the front corners. The output shaft had also been fitted with a flexible joint but this was in poor condition so I restored it to it's original configuration.