A deceptively simple but not very secure security key

I recently recalled an interesting security key for a building car park at a block of units we stayed at a few years ago. It was a small block of units with maybe 10 – 20 units in the whole complex, probably built in the 1970s. Underneath was a carpark with a roller door to prevent unauthorised access. A small box stood near the entrance with a receptacle for the key.
In other places we have visited we were usually given a remote control to gain access or sometimes a card to insert. This place had a 6.5mm audio plug attached to a key ring to insert in a socket near the door.

The key

I found it particularly curious as the plug only had two connections, that is it was a mono plug. I wondered how they had implemented security with it. I unscrewed it to reveal a deceptively simple circuit… a wire.

With cover removed and circuit revealed

I have to give the creators full marks for simplicity, but not so many for security.