There is no "overdrive" in that trans. All MB transmissions till the 722.5 were straight one to one out the back hi high gear. The 722.5 came first in the 129 body 300SL (I think). It then also appeared in the 140 chassis 6 cyl cars.
If you are in high gear then you are doing the right rpm. You might suspect either gauge if things aren't in order. The trans is going to have one turn on the drivesahft for every turn of the motor in high gear. Anything less would be slippage and that won't last long!
Bosch Master, ASE Master, L1
33 years MB technician