If it is a home built, then someone put an astronomical amount of time in engineering and machine work just in building the breech block assembly much less the entire assembly. The transverse and elevating knobs look like military hardware but it strikes me me as something you'd see on a prototype or Mark I version. If an individual built this from scratch they would've been far smarter to copy the transverse and elevation mechanisms from the M3 37 mm, i.e. to design work has already been done - one only has to miniaturize it as the M3 is far more efficient and user-friendly than we find on this piece. All of that taken into consideration we have seen some very clever engineering and machine work by folks who miniaturized the Browning 30 caliber machine gun into a .22 caliber belt said device.
Britishicon 2 pounder training device: it's been my experience in collecting over 50 years World War I and World War II small arms that training devices are usually the exact same dimensions as the genuine article with the exception of being sub caliber. However, I have not found a good picture of a towed British 2 pounder for comparison.
Please keep those ideas coming.

Further, the point was very well stated, what's the point of a device like this when the M2 MG was available and would obviously be for more effective and useful. So on the final analysis, it could very well be a home built.