Well, David, a couple of points.
Firstly, in terms of the rear bench seat - I`ve seen two others, and one had armrests and the other didn`t - so take your pick!
...and then, talking of the backs of the front seats - dead simple. They had the metal spring units like in a mattress, with fabric wadding over that (presumably horse hair originally) and then the leather. I would imagine that a bit of autojumble work would find you some spring units as they must have been a standard supply item.
Hope this helps.
Terry
PS Thanks to Barrie for posting my photos. I ought to say that the semi-tonneau in the middle one is not something I`m happy with. We have been due to make another, better fitting one, but the trimmer`s wife has been quite poorly, so he`s not yet got round to it.

Thanks terry. I am looking at getting a couple of seats, and modifying the springs to fit. I was hoping to get some pics to understand how it is arranged, especially the top as it tapers.

Having looked at the pics of your car, with regard to the bench seat in the back, it looks like ours has retained the curved side panel which takes the hood frame, which seems a bit odd, as there is no frame, but would make a good spot for an arm rest. I think I will do a little remodelling on the basis that it ought to be finished properly. That should give an increase in width, and somewhere to rest an arm !
Thanks
Db