Would think this is a refit of a 5-speed auto?
Mercedes introduced a 5-speed automatic in 1990 on certain R129 and W124 models (per Daimler media website.) Maybe it's one of those?
Alternatively, perhaps, a fitment of a 3rd party unit in the same way that some have placed Getrag and Tremec manuals in the Pagoda?
Just guesses here, and not certain they would reveal their "secrets"...