I'm pretty certain that's a wheel from a (very old) Sankey trailer! I had one of those on my Series III when I first bought it, too. Just did a bit of Googling and Nagbol were bought by GKN Sankey in 1973 apparently, but with a flat centre flange like that the wheel is likely to be much older...

Thanks very much indeed to both of you for your replies. James, what sort of figure do you think would be normal compression-wise? I was sort of expecting somewhere around the 250 psi mark or is this being unrealistic in practice? Re-ran the test with minimal improvement after doing a top end...

Well, everyone has to start somewhere. Doubtless I've broken every rule by starting a thread about something which has surely been discussed elsewhere, and if so I apologise - my search capabilities leave a lot to be desired. To the crux of it then. I have a 1981 Series III 88" with a 2.25...