AT&T used to use a "miniature" 8P8C that was keyed at the top like that. The large operator consoles for the Merlin Legend had an expansion port for an additional button field (sidecar). The connector for both looked very similar and would also not accept a standard width plug. I would fix broken ones by crimping a standard plug on and then take a minute with my Dremel to shave down one side. Ghetto...I know. But on those, I think the key was just on one side--not both.

I also feel like this could be some old serial or IBM thing. The family of DEC connectors were a bit odd too, but only because the key skewed to the side. Either way, the housing is clearly designed to keep the riff-raff out!

"Press play and record at the same time" -- Tim Alberstein