Hi,
I'm in the middle of retro-fitting air-con to my 740 diesel. I've swapped the entire heater box and got everything fitted bar a couple of A/C hoses (which will need to be fabricated/altered) on mine now.
Your mystery box looks similar to, and has the same number of vacuum hoses as one on my diesel - but my diesel has two solenoid valves in total.
The one that looks identical to yours has the following connections:
Electrical connector - top right of it in your pic.
Vacuum hose - bottom right of it in your pic - connected to inlet manifold and second solenoid valve
vac hose - opposite electrical connector, top left in your pic - connected to injection pump.
Vac hose - opposite and below electrical connector, bottom left in your pic - connected to interior lamp (for supplying air to flow over temp sensor on ECC system).
I have no idea how similar the 740 and 240 systems are - I'm guessing 240s only had the manual A/C rather than the automatic climate control?
Therefore the above info may be no use to you at all...
I wish you good luck in doing this - I've found there have been some challenges in mine
