So we all know the story: when the tech closet is cleaned for the first time in ten or fifty years, you find some things that you have very little clue what they are.
I found a thing like that.
It appears to be an RCAadaptor with a mini 5-pin XLR end.
Yes, probably a tape deckconnector. Interesting stuff, I have a Pioneer SX series receiver and it has a DIN connector on the back for a tape deck. 2 pins for L/R Playback, 2 pins for L/R Record, and a fifth pin for grounding the rest.