I have had a reply!
It seems he is preparing a new batch of the GSR 800 kits, so the potential customers that I referred to Des could end up happy. He confirmed he had received their e-mail separately and had replied to them individually.
I'm not sure there were a limited number of prototypes. If Irish locomotive history tells us anything, it is that there were a great many locomotive classes, albeit many (most?) only containing single figures.
I wondering how the spring carrier behind the iron is supposed to run straight if there's no allowance in the iron itself for the bearing to run up and down.... Bill generally put in an oval hole so that the bearing could be guided. It looks like he forgot to design it in here.