Date registered: Dec 2006
Vehicle: 30+ mercedes
Location: Long Island, NY
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you've got two problems here. First, the month of manufacture is stamped on a plate in the door. Second, the 1974 280se was not imported to the US, so you won't find the parts so easily.
The good news, is that anything not motor related would be identical to a 280S. The bad news, is that for motor parts, contact the mercedes classic center or polish up your ebay skills and look in Australia, the UK and the rest of europe.
The 280E and 280SE cars from 1973 to 1975 were popular and powerful straight 6 motors. The fuel injection system is called "D-jetronic" or just "D-jet". It's very similar to what was under the hood of the 450se in those years, but with a different cylinder arrangement (since you're missing 2). The parts from a 4.5 can not be interchanged.
If you want the service manual for the motor that specifically covers the d-jet system, you need to find the loose-leaf copy of the M110 manual or you need an early edition of the paperback book. Ask any prospective seller on ebay with an M110 manual if it has section 7.4. That's the one you need. That old paperback is thin, like about half an inch thick. Later editions balloned to until they were two 1 inch thick tomes. Mostly because the M110 motor was used for like 16 years in various forms.
PS. Lovely cars btw