It's the DJI Ocusync Air camera and transmitter for their FPV Goggles RE system. They discontinued it in favour of the new FPV Goggles V1/V2 system, which is very nice, but also quite expensive and not ideal for the purposes I'm using the current one for.
The supply of transmitters, and even more, cameras, seems to have dried up entirely in Europe, and/or become very very expensive. The last time I bought a complete Air set it cost me about 150 quid, however the last time I found anyone with just the transmitter on its own for sale they wanted something like 175 pounds for it plus shipping and so on. The cameras are virtually impossible to lay hands on.
I've got two sets, and ideally I wanted a couple more, but outside north america you can't find them at all as far as I can see. They're still in stock in a few places in the US, but I haven't seen any in Canada yet although I'm still looking.
Experience is what you get just after it would have helped...