I have recently come across this airmail cover. It has a dumb cancel from ? the date is also unreadable and a London (UK) F.S. mark on reverse which is dated 19th August 1943.
There is a Canadian Naval censor cachet on back and also two censor labels. One Canadian and one English.
What interested me was that the cover appears to have been registered in London before being sent on to Glasgow. Note the blue lines cross both censor labels. There is no indication that any fee was charged for this registreation.
Why would this have been done ?. The only reason I can think of is that something was included that the postal authorities (or the censor ?) in London thought was important / valuable enough to justify the registration.