https://en.wikipedia.org/wiki/Overdrive_(mechanics)#In_Europe
The basic design is the same, although the modern GearVendors units certainly have a higher torque rating than those found behind Volvo Fours.
The J type came first, the P type was the last design.
GearVendors bought the overdrive, lock stock and barrel. They are the sole manufacturers for a while now. They have uprated its torque handling capabilities.
It's a planetary gear arrangement which is hydraulically activated, like an automatic transmission.
I wouldn't advocate trying to adapt a Volvo unit to your hot rod.
R.