@blankideogram @natecull also the use of the "security pin" isn't clear (although it suggests "security" in the context of "springing a catch that prevents heavy metal thing crashing down hard on the worker" rather than keeping out unauthorised persons")
I always get the impression that the folk who do this tend to be former employees of the marketing companies who have ways of getting the posters printed cheaply as well as inside knowledge of how to get into the cabinets etc..
@blankideogram @natecull many UK news sites are running articles about the kids who produced the posters (they initially only made them for their own use, the SPG lot paid for the rest).
looking closer at the info the "security pin" may be for use instead of sticking a finger into a bit where there could be sharp edges.
I'd suggest anyone attempting this practices opening and closing one first (before altering content) and to remember some of these ads face the road, so watch for traffic!