For info: I had been trying for 6 months to find a distributor cap for my alternator 1959 B31, Distributor 18D1, part number for cap 424151.
I must have tried every parts shop and was getting concerned that I would not be able to source one. A tip off led me to the 'Distributor Doctor' tel:01984 629540 (the advertise on the front page of the Old Bike Mart), they had stock and it was delivered a few days later at half the cost that others had been selling them for in the past. Highly recommended!