Thanks for your elaborate answer and the compatibility list.
That list indeed lacks the B200 application.

Previous to my current 245, I had had another one for about 12 years. That car was identical to the one I have now in every respect. It was also registered in January 1992 and a look at the VINs of both shows they are only 918 apart.

I found some photos I took of the ECU of that previous 245. Surprise: it had the -936 ECU, and although the label of the EZK isn't fully visible, I can just make out it must be different too. The current one being a 0 227 400 176 and that one a 0 227 2?? ????.

Hmmm...I wouldn't be surprised if a previous owner swapped in a ECU/EZK combo from a different car. The connectors of both show signs of measurements taken the wrong way and when I had just bought my current 245 I found a wire had been added to make both fuel pumps run continuously as soon as the ignition was switched on. I also found two spare main relays.

Today I went for a hike at about a 45 mins drive away from home. I took the opportunity to measure some of the signals going to/from the ECU and EZK while on the move, stopping a few times to change the measuring leads.

Everthing looked fine, but I found that "IAC" is a misnomer. That valve is adjusted all the time, not just when idling.
I experimented a little and found that it adjusts the IAC according to certain conditions.
At cruising speeds and part throttle, I measured voltages of around 7.6 V. Only the speed seemed to affect it.
Idling at a standstill sees something like 8.6 V and when you engage the clutch without touching the throttle that voltage drops to compensate for the load dropping the engine rpm. The IAC is turned open to allow more air into the engine. So far so good.
Then there's slowing down with the clutch engaged, the voltage slowly rises as the speed drops. But when I then depress the clutch, it quickly rises to a normal idle voltage, vehicle speed being no longer a variable to be considered.

So the ECU must be able to ascertain whether the clutch is engaged or not, does it determine that from the AMM?

At moments when the engine idles high, depressing the clutch and coasting does not see the sharp drop, but a gradual one and a last one when VSS shows the vehicle has stopped. It's as if the ECU thinks your engine braking rather than coasting.
The other function of the IAC is to control the intake vacuum, hence it's always adjusted by the ECU. I knew that but I didn't expect it to be adjusted while cruising.

I'm still unsure as to what's going on. But while testing the knocking sensor by almost stalling the engine (foot on brake and slowly engage clutch until it almost stalls then depress it quickly) I found that often the idle returns to 1000 or even 1200 rpm.
These are nice round numbers, so it seems more and more to me that the ECU performs a deliberate function, for the wrong reason.

Sorry if this is getting a bit messy and complicated.