There were indeed once 20 inch rims, they were quite common at one stage.
Norton Internationals used them on the back wheel into the 1950s even.
I'm presently chasing a 20" tube for one, WW2 Enfields used some on the front wheels.
You can still buy 20" tyres too, Michelin and Mitas have both supplied them in the recent past.
But the tubes seem to be a scarcer item, and I'm reluctant to use a 19" stretched to fit.
Cardan will probably chime in here maybe, since the subject of Douglas using odd sized wheels
and optionally different sizes seems to be a persistent theme through their history !
Quite an elderly 20" tyre, marked in both systems, well worn.
Bit hard these days, but suitable for display - and that diamond tread pattern is now rarely seen ?