To my knowledge, there is no such thing as a 1979 300DT. The 300DT wasn't manufactured until late 1981. This guy claims that it's a 79, but the headrests and everything else sure don't look like it. It smells a little fishy to me, no engine pics, etc... It could be a conversion however.
http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&category=6330&item=4524257719&rd=1

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'81 MB 300SD, '82 MB 300D Turbo (sold/RIP), '04 Lincoln Town Car Ultimate

Sooner or later every car falls apart, ours does it later!
-German Narrator in a MB Promotion Film about the then brand new W123.

Wow, that is a REALLY oddball car. Has alot of euro options..

The Climate control makes me think it is a 1981+ car...

The numeric-only VIN is consistant with a pre-80 car. Never seen a '79 W123 like that though. Ask him for a door tag photo maybe?

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'78 300D 310K Maple Yellow / Bamboo
'87 300D 81K Barolo Red / Safron (RIP @ 113K)
2003 E320 34K Pewter/Charcoal
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What's up with the badge on the grill? Maybe this could be some kind of prototype car

He also describes it as having "rack and pinion" steering. Never in a W123. It sounds like it was his German mechanic's hobby-mod car, and he was sold a big fish story along with the car.

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'78 300D 310K Maple Yellow / Bamboo
'87 300D 81K Barolo Red / Safron (RIP @ 113K)
2003 E320 34K Pewter/Charcoal
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Hmm. A Frankenwagen.

What is the black thing underneath the ignition?

Nothing on that car makes sense. It can't be '79 as that ACC wasn't until 1981. But the vin does point to a 1980 or earlier car. The unlabeled switch above the ACC panel?

He says he bought it from a trained German mechanic, so I will guess that this is a custom job.

How hard would it be to swap the old ACC system for the new?

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'90 300SE 298k
-300K and it gets put into retirement.
'80 300D 255k Purchased new by family in 1980.

Had a:
1973 220 (gas)
1980 300SD
1992 400E