We’ve met a weird transmission recently.
Mercedes used the Hydrak (hydraulische Kupplung) in the late 1950s. Like the Saxomat system with which it shared part of its design it used an electrically controlled and pneumatically operated automatic clutch for gear changing and in this case a fluid coupling for starting. To change up or down you just lifted the accelerator and moved the gear lever (mounted on the column in this case because 1950s) whereupon a switch activated the clutch for you. To start and stop you just used brake and accelerator. Why not just use a conventional automatic? Because the Hydrak was cheaper to make and was being sold to buyers unfamiliar with automatics so they had no frame of reference.
In the event it turned out to be a failure, being both unreliable and subject to driver abuse, and used cars with it were so hard to shift that for years afterwards Merc dealers used to carry manual conversion kits*.
Mercedes abandoned the idea but similar transmissions were used by VW and Porsche for many years afterwards.
*containing this little lot, as per www.mbzponton.org
Clutch release bearing
Complete bell housing with release fork and cover plate and any other items attached
Transmission input shaft
Transmission cover front
Front half of drive shaft
Clutch pedal with linkage and return spring
Annular nipple (vacuum fitting on brake booster) you could plug the line to the vacuum container.
You may need to change the alignment pins between the engine and bell housing.