Very interesting--thanks for the photos. Those tubes look very uniformly made to me--quite similar to the factory mirror arms used throughout the entire production run, with fixed clamping nuts on the sides. I would suspect they have a drain hole in the bottom--if not, one would be suggested. I cannot see a nut to fasten the lowest of the three attaching bolts--maybe it's been captive-welded inside the tube itself, or is it simply a sheet metal screw through the side?
And yes, the bumperettes came with chromed diamond-plate attached to the top surface, by truss-head Phillips-headed bolts, into small welded tabs on the top-surround of each bumperette. The diamond-plate slightly curves up on the top rear edge of the bumperette. Unlike the FJ40, the bumperettes were intended to be both a minimal bumper AND a step to access the bed of the truck. Here in the USA, there were several companies that produced different varieties of very heavy full-length bumpers, sold on the aftermarket, and many of the existing trucks here today have, or had, them installed. By the way, the FJ45 pickups stopped being imported into the USA at the end of 1967, although later versions came into Canada as late as 1978 or so. Where you are, the entire run through 1986 will show some interesting variations over the years.
For your year model, although the correct factory rear valence reflectors are no longer sold, here's a tip: the exact Koito-brand reflectors are available by buying new pre-1979 FJ40 rear-side reflectors (with the chrome ring), disassembling them, and discarding the rubber surround and the internal bracketry. An expensive method, but it gets you the original part.
Oh, those upright bolt holes that were pre-drilled in the upper rear sides of the bed, originally came with plastic plugs for the trucks not having the hoops for the canvas cover--which is most of the trucks produced. The holes were used in the factory hoop setup to secure the rear supports for the factory channel that sat on top of the bed-side rails. There was a long flat bar that inserted into each hollow rear-bedside corner, to be bolted with two bolts per side. All of this hoop-business was rarely seen throughout the world unless specially ordered. There may have been a special run for some military order (Pakistan comes to mind), but the records for that were likely kept in-house at the factory. The specific history of your truck would be interesting to know if you could find the original owner and the order placed for the truck--I know, not much of a chance.
Since your setup is on the rare side, you may have to settle for a used rubber tailgate bumper to match the extra-large one you already have. The smaller rubber bumpers are just that--too small--but a resourceful person could probably use them with some added rubber extensions. And while you are at the junk-yard, you might get lucky and find a hoop set for the canvas cover, if you're interested. There were several huge junkyards over in KSA some years ago that might just have such a thing-- can be hard to locate, since the junkyards were hidden behind tall walls and fences--for "aesthetic" reasons.
Lastly, I would suggest removing those side hoop tubes to do a really clean job of removing old paint off the bed sides. It looks like your workmen are using liquid paint stripper--if you have access to a sandblast cabinet, the tubes can be totally stripped of old paint and rust, repainted separately, and reattached with new bolts on final reassembly for a clean look.
Thank-you for taking the time to document this. I may be the only person interested in any of these oddities, but those of us in the rest of the world never saw some of the things that may be common to your country. Truly, a picture can be worth a thousand words--so keep showing the rest of us the progress on your truck !