In the military sense, the phrase ‘posting a guard’ means putting a sentry or sentries out to keep watch of an establishment or a camp to prevent any unauthorised personnel from entering the area and taking the men within by surprise. This common phrase however inspired one comic postcard artist who came up with an alternative, and very literal interpretation of the words:
Here a member of a Guard’s regiment is being, quite literally, posted by being fed head first into a post box. The humour is rather weak to modern eyes, but must clearly have appealed to an Edwardian audience. The soldier doing the posting is clearly from a Scottish regiment and the grey uniform suggests a territorial unit. My best guess is that he is meant to represent a member of the London Scottish who were known for wearing a uniform of Hodder Grey:
These sort of postcards are very hard to date without a postmark on the back, but the style of illustration and the uniforms suggest very strongly that this was produced before World War One and so to say it is Edwardian seems correct to me. The humour might not have dated particularly well, but the card has a lot of charm about it.