Stamp vending machines were first introduced in the UK in 1907 (during the reign of Edward VII), so it must have been a later addition to this particular box, but I've no idea when. However, my best guess is sometime pre-WWII as the wording suggests a single stamp was issued for a single coin (probably a pre-decimal penny coin).
I noticed this curious little ghost sign on a VR post box on Crow Road in the Broomhill area of Glasgow a few months back, but only got round to photographing it today. It's made of enamel and its message (Await Delivery of Stamp Before Inserting Further Coin) seems rather odd, until you realise it's all that's left of a stamp vending machine which used to be attached to the post box.
Cont./