On mine I just cranked on the nut until it was tight, and that gave me reverse. I think the bolt just went along for the ride. The nut was likely frozen on the bolt just as yours is. A couple of weeks ago I did a search on eBay and found an adjusting bolt that looked just like the one on the picture. I think the key works I used were "722.1" and "transmission". You might give that a try.

It is also possible there is more wrong with your transmission than just the adjustment of the band. If you can find a good Mercedes mechanic they might be able to figure it out for you.

Hope it works out for you.

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Sedecrem

82 300SD 290k(needs engine swap)
79 240D 354k with new heart transplant w/220k
82 240D w/auto tranny soon to be manual 4 speed