Closeup pictures of how the tailgate latches.

Not sure if this is original or not, but these cigar shaped pins are bolted to the tub sides, then protrude through the tailgate edges. 

A hole in the end of the pin allows securing the tailgate.  S-hooks were used originally, I used these spring-loaded clips (Caribeaners?) instead.  Using these at the top, and a a threaded link on the attaching end I can remove the chains what the trailer is stored outside, or when going down the road empty.  The trailer bounces quite a bit, and the chains flail around, make noises and paint chips etc.

Note the welded-on washers are not original, I think the way I reinforced the bed sides spaced the pins wider apart, so I needed to add the washers to make up for that added width.