1977MB300D

My new project car, 1977 Mercedes 300 D naturally aspirated has a problem going into reverse and first gear. I heard once that reverse and first share a band in the Automatic Transmission and that it has an external adjustment. Has anyone else heard this, and if you have, can you tell me where the adjustment is?? I know I will likely need to rebuild the transmission, but if I can squeek a little more time out of this one.. it would be great.

Thanks! Im new to the forum and loving it!

Capt Wendell