I bought the second set that was on ebay, that Chris brought to everyones attention....yes for the noventy value only ...before you ask. Three of the four tyres are in appearance still good enough to use but I wouldn't trust them, because they are indeed over 30 years old.
So what I did was cut the worse one open for our NEC clubstand display. The purpose being, to show what they are like inside. However I opened it up, only to find that the canisters that should have been inside, had been removed and judging by all the sticky stuff on the inside the tyre, it obviously had a puncture at some time. The tyres are Metric by the way, hence the odd ball size. The rims actually split to get the tyres on and are bolted together like old-time commercial wheels. The tyres themselves have a reinforced side wall and the bead is a lot thicker than normal tyres and is trapped in place when you bolt them up. The one thing which is evident as soon as you go near them is the weight. They must be nearly twice as heavy as a normal wheel and tyre. The wheel trim doesn't fit a normal wheel either because of the oddball size and neither does the wheel nuts, they are odd as well.
For those who don't know about the P6's fitted with Denovo wheels, they had a different boot floor because there was no spare wheel when you had Denovos fitted, you only had the four on the car and a carpeted boot.
I am going to take the others apart soon to see if any of them still have the sealing system still in place, if they do I will publish pictures.