Thank you! Well, given that my '81 300TD has a 300SD drivetrain, it must have the equivalent cruise control elements. Here is a page I found useful...

Professionally remanufactured parts with extended warranty

So, it seems like it might be the 005 545 12 32 amplifier!

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[GONE] - 1995 Mercedes E300 Diesel - 130k miles - Smoke Silver (702) over Mushroom leather (265) - Bladder blasting, coast-to-coast work machine.