The way I understand it "discs" were never stopped here in Great Britain, they transitioned from the civilian disc with the price field left blank to the Defence of the Realm or "G" disc, Army Form A2017, these were used with Army vehicles and civilian vehicles performing Army tasks, I have seen written orders that they were not to be displayed on Army motorcycles, unlike the RAF version, but were to be attached inside the AB412 & AB412m log book, but I haven't worked out exactly when the change happened, hence my interest in the one on Lex's Big4, I also understand that machines were exempt from this disc in the run up to Normandy if they were with invasion rather than home units.
I think this may explain the rarity of the early alluminium holder and the commonness of the later RAF blue versions.
