Well, I've eventually got my own one. Seems to be a slightly updated version - with a much longer battery wire (rear wired), and terminated already with a deans connector.
A quick plug into a tank, and a worrying hissing noise from the box indicating a leak as I turned up the pressure with the reg. As soon as I disconnected, got the pressure up on the regulator first, and reconnected the leak has gone - I put it down to the seals or something not being seated fully, and needing a bit of a 'jolt' of air pressure to rectify. I've not popped it in a gun yet, but it seems nicely functional so far. I can confirm that the control unit needs an 11.1v battery to operate - doesn't function at all on a 7.4.
I thought MP5Ks used a V3 gearbox? In any regard, I was originally considering one of these for a (full fat) MP5 that uses a standard V2 gearbox case. But in the end, decided that it might not work - my logic being that it doesn't seem that the nozzle can be swapped out/nozzle length adjusted - so I think you are stuck with an M4 length nozzle (albeit I'd love for someone to prove me wrong on that). Potentially also a little bit of inventiveness would be needed for the selector plate.