Very nice usage of a formula, also with cancel commemorating the day that the UK went decimal. Does anyone have the book that might tell us whose etiquette/stamp that is?
Some may have wondered if there are errors in aerogrammes. Here's one from Hong Kong:
The dark blue is entirely missing. Compare with the normal one below:
More commonly found are miscuts, where the die cutting of the aerogramme is inverted in most cases. Stuff happens with the production of postal stationery, so those errors are scarce but not usually rare. The missing color here or miscut noted could still be used, so perhaps they were just allowed to pass through inspection.