I have been cleaning a bunch of couplers lately. One of them is like that (has a plug). The others are fully machined with no chance of coming apart like that. I have one early coupler, but I can't remember if it's for manual or power steering.
Interesting! I've never seen one like this. It's almost like the plug was just pressed in. Good to know it's been seen before, at least. I'm just going to epoxy it back in, seal it up really well, and try to forget about it. I guess as long as it doesn't leak, I'll be good to go.
