i know for sure that the C500's had a trans brake for that
this may too

on the c500's its a small brake drum on the engine side of the tranny towards the bottom

look there
i would guess its shoes
idk if disks were around in any sense when this truck was made lol
its either that or an exposed drum and lining (with the shoes outside the drum)
follow the linkages you might have to get under it depending on the clearance and the size of your head you might have to jack it up lol
be careful
use jackstands or blocks
even then ill try to use a mirror and drop light as much as i can