It is my understanding that a driver side arm rest was standard equipment on these trucks with the Custom Cab option. I think a passenger side arm rest was either a factory (or possibly a dealer) option.

These arm rests double as door pull handles and attach to the inner structure of the door via a couple of screws and clip-in nuts. The clip-in nuts fit into square holes that are present in the inner structure of all these doors. The screws pass through a couple of 1/2" diameter holes in the inner metal door panels. Here is a template that I drew based upon measurements taken from a 1965 Custom Cab truck with an original driver side arm rest.

Note the 1/16" difference in the hole heights is due to the fact that the edges of the panel are slightly rounded out in the centers.