The early HydraMatics used a parking pawl on the output shaft that engaged when the shifter was moved to reverse with the engine off. Beginning in 1956, the Controlled Coupling Hydramatic shift quadrant included a Park position, but that transmission was never used in trucks.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!