This card is the only one in the set with a different back. The card refers to the fact that they have Royal Warrants to the King and Queen and the Prince of Wales. The King and Queen were King George V and Queen Mary. The Prince of Wales was Prince Edward (King Edward VIII). He was Prince of Wales from 23 June 1910 to 20 January 1936 when he ascended the throne. The next Prince of Wales was Prince Charles on 26 July 1958. King Edward VIII abdicated the throne in the same year 1936. Your listing shows the first use of this card as 03/08/1938. While printing may have taken place in 1936 I would be surprised if a firm with Royal Warrants would print this information after 1936. This, therefore, suggests that the card was printed earlier than 1938. You may wish to enter a note to this effect on this entry.
Interesting, I believe Richard and Alison, only put a first use when they see it mailed, and it’s only a rough estimate as it applies to the whole set:
That’s where the 1938 was found.