If you have one of the old o-rings and can get some dimensions, like the thickness id and od I can check a catalog and pick out a number for you. Its probably a metric size, the o-rings come in standard sizes so I would select several sizes if in doubt. You could also measure the shaft size, bore, groove width and groove diameter and select a size from that.

Probably what you will find is in this size someone will want to sell you 200 for $20.

I bet webra produced some parts out of spec, so even a new o-ring might bind unless the groove size is corrected.