The slotted bolt is more likely an adjusting screw with a lock nut. Been 20+ yrs since I worked on a box, but I would make damn sure its the filler before "screwing" around with it. If its not leaking, its fine, unless it leaked itself dry decades ago.
Get a factory service manual so u have a clue what you're working on.
Bishko has reprints that are better than originals.

Plastic trucks suck...
Iron trucks rule...