D15 Fuel Tank Under Seat

It looks like all the D-series I've looked at in photos have the fuel tank in the rear and the filler at back of right rear fender. Well, mine was under the seat with the filler right behind the passenger door. Was this factory? There is a nice factory looking hole for the filler. Maybe because mine didn't have a bed (just a homemade flat bed when I got it), so they didn't put the fuel tank exposed at rear? The tank just has a big flange all around and rests in the big cutout at bottom of cab (with a couple of screws).

What's under the seat for you guys that don't have a fuel tank? Is it solid or the opening? Not sure I like the idea of sitting on a fuel tank. Thought about side mount tank under bed on left side.

This has been a topic here before! All my D-30s have had the tank under the seat. The consensus here I think is don't worry too much about it. I don't worry about it, but I don't smoke, either.
LOTS of old pickups and trucks had gas tanks under the seat or behind it.
My D-2 just has open space inside the seat "pedestal". I've never seen a D-15 before in the flesh.