The mechanical parts inside indicate that it vended coins or possibly tokens, never seen one like this before.
Looks like you would put a 6d in the slot and got?
Anybody any ideas what it was for?
That was my first thought but it's not likely. The K4 had a stamp machine and a letter box but they were built flush into the kiosk for security. More likely this was for a,GPO coin op phone but not one mounted in a kiosk, perhaps a hotel, pub, club, or even more likely, an underground station, where it could serve more than one type of machine where it was at least partly kept an eye on. Am I right in thinking that there is a coin chute for the 6ds going out of the bottom of the machine?