The 4 speed semi-automatic is the M-6 pictured above, which will not have a flywheel but a "fluid coupling" as shown above with the starter ring gear, which is similar in looks to a torque converter.
In the case of the 1950 Dodge though, it does not multiply torque, hence the name fluid coupling.
FLUID DRIVE is really just an advertising term to refer to the fluid coupling.
The 50 Dodge May or may not have the M-6 depending on the model...the base model Dodge would have the regular 3 speed, and the "upscale" Dodge would have the M-6. If you roll back the front mat there is an inspection cover on the side of the trans hump, if you open that cover and you see the governor and solenoid, you have an M-6 semi-automatic
49 Chrysler shown but 50 Dodge floor access will be the same.