The tub floor on a landrover is an alluminium sheet that is riveted to some cross struts which you can easily see from underneath.
I don't think was anything fitted to the floor in 1984 ?
If you drill out the rivets you can ease the floor out, i believe that YRM do a 2mm or 3mm replacement.
In the early S3 there were no door switches for the interior light, the switch on the right in your pic is for the interior light and the dash lights.
The switch on the left is for the heated rear, safari door, window.
If you have a pick up/open tub now they typicaly came with a (drop down) tail gate, if the rear door is side hung then it's likely to have been cut in half when the top was removed.
Original pickups often have cleats along the rear tub capping when the tilt was tied onto.
Post some pics ....