I'm digging this thread up because the original question was never quite answered. I also wish to add additional information I recently discovered which may help others who own cars that use the same 55 amp alternators (Bosch AL-80-X).
That "thing" mounted to the RH side of j-galap's engine compartment is (was) a external voltage regulator (VR) used in conjunction with the 55 amp alternator that came with '74 240Ds. This was an upgrade from the 35 amp alternator that came with '73 and earlier W115 220Ds, which also had external VRs. The external VR only came with earlier W115 240Ds. Have a look at this page in our sponsor's online catalog:
https://www.pelicanparts.com/catalog/SuperCat/3195/MBZ_3195_ELCHRG_pg1.htm#item4
Note that both internal and external VRs are offered. There is a chassis number break corresponding to numbers 056705 and 056706.
My '74 240D came with the external VR setup. My car's VR went bad recently, and I replaced it, but not with another external VR. I instead chose to replace it with the internal type VR associated with later cars. I simply replaced the brush holder attached to the alternator with the new style "short brush" VR purchased from our sponsor (Bosch). It works fine, and simplified my car's engine compartment a bit. Here's what the brush holder I removed from my car's alternator looks like:
So apparently what happened to j-galap's car is the alternator was replaced with the standard Bosch AL-80-X replacement listed for the car. Those alternators come with the internal VR so there was no need to use the external VR.
I also want to add that during my research into whether or not I could install a internal VR in my car's alternator I found a pretty useful web page. The page is part of a Volvo 240 related web site, but those cars and others used the same 55 amp alternator. The owner of this page sells adjustable output VRs, both external and internal, that might come in handy. There's a a lot of valuable information about alternators in general on that page, and I found the confirmation I needed before ordering the internal VR for my car.
https://www.240turbo.com/AdjustableVoltage.html#externalregonly
Cheers!