The SE2a was a development of the SE2, the main difference being the addition of a reversing engine. This is one with the earlier "straight" reversing lever, made from a substantial steel bar instead of the later, thinner, pressed steel. My attention was initially drawn to it because it had some interesting Meccano attached to it and I then saw the rare type of push button whistle.
The burner is a Mamod standard vapourising meths type. The boiler is 1 3/4" dia x 4 5/8" long. It has the standard 1/4" safety valve. The whistle is of a rare type which was a short term transition from the all brass tapered valve type and the newer spring loaded lever type. This one has a spring loaded valve but it is operated by a plastic push button on the top end.
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 5/16" dia x 3/4" stroke. The exhaust is fed via two pipes, one for each direction of running, to the base of the chimney.
The bottom photo shows the engine in it's "as acquired" condition. As mentioned, it came with some Meccano parts, which I have removed and added to my Meccano collection. The engine was in reasonable condition, needing only cleaning oiling and running in. The boiler fittings were furred up with lime scale, which was easy enough to remove with vinegar. There is slight rust on the base and a bit worse on the firebox. I'm considering repainting but that for the future, if atall.