"the hydraulic tank should be on the other side of the truck"

This here (workshopaholic.net/forumImages/_DSC6430.jpg) ? If so, the previous owner was confused, because the left-side tank is half-full of hydraulic fluid and this tank is empty. Judging from the "type G" on the data plate, apparently this forklift started out as a gasoline model and was retrofitted for propane, so the left-side tank is just left over.

Also, there's no dipstick attached to the cap, an idea how far to fill the tank? Or a part number for the cap (although I bet that's frighteningly expensive).