My friend has an exterior door with this strike plate on its frame. Does anyone know what this type of plate is called, and what purpose it has? The inner part is hinged in a way that allows it to rotate around a vertical axis by just one degree or so when the door is latching. It looks sort of like an electronic door strike plate, but based on its apparent age, I doubt that's what it is.
If you think electrically controlled locks were invented in the 21st century, think again. That's certainly what comes to mind when looking at it, and likely it rotates more than 1 degree (and lets the door open with the latch sticking out) when some solenoid is activated, if wired up and working for that purpose.
"With the latch sticking out" is why the frame is cut away in front of it (to the left in picture) which is, in fact, diagnostic. Rather than try to get building control wires to work on a door that moves, the strike on the frame that does not move is controlled by the building, but someone with a key (or hitting the exit bar) can still operate the door latch, as needed.