Should be a tag somewhere on the cap itself (unless PO removed it), if underneath, by the lock mechanism, or usually somewhere along the outer edge, if not, could be in one of the "bowls" of the underside. Another possibility is on the outside of the cap on the flange at front of the cap (facing the back of the truck cab). Main point being, you may have to look this thing over with a fine tooth comb to find it.