Great runner, pulls a lot for its size. I always assumed it was scratch built using Wilesco cylinders.
But:
Lately I bought a Märklin R4000 (tender) and its sibling T4000 (tank version)
The tank version looks suspiciously similar to the homebuilt loco:


It seems to be made using wheels, axles, cylinders, frames and boiler of the T4000 with the cylinders moved to the other side and a set of longer rods.
Mystery solved.

