1966 A904 loses all gears overnight

In the business that is called "Morning Sickness" and is caused by hardened up rubber seals.
Hardened up seals don't flare out like they are supposed to, and don't seal against the drum/piston surfaces.
(Meaning a soft parts rebuild at the least)

If the fluid is burned it probably happened from trying to make it go anyway.

Converters only take about 5 seconds to refill from drainback.