From the 123 Diesel owners manual, p.67 in the copy I found online (too lazy to run outside and grab mine) this looks to be a late 70s 200D/240D/300D version, but I would think it's the same for yours.

"Check transmission fluid level with the engine idling, parking brake engaged, and selector lever in position "P". The vehicle must be parked on level ground. Before the check, allow engine to idle for approx. 1 to 2 minutes.
Complete cleanliness must be observed! To wipe the dipstick, use a clean, lint-free cloth (or better still leather). To fill the transmission with fluid, only pour it through a fine-mesh filter into the dipstick opening. Even the slightest impurity may cause operational troubles.
The fluid level in the transmission changes with the temperature. The markings on the dipstick (max. and min.) refer to a fluid temperature of 80C/176F - normal temperature with the transmission warm.
At a fluid temperature of 20-30C/68-86F, however, the maximum fluid level is about 30mm/1.25 in below the dipstick minimum marking. This information serves as a guide for fluid change which is generally carried out at this temperature.
The fluid level should not exceed the upper mark and any surplus oil should be drained or siphoned off."

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech