Help on identifying light socket? (Japanese apartment)
IMHO this is a somewhat low-quality answer just a cut above link-only (which is why it was posted as a comment), and based entirely on "choose the right keywords for a search about an item I know nothing about directly." But since at least two people have requested it as an answer, here it is. I encourage anyone who actually has direct experience of these or who has some in their home to answer.

Appears to be #15-18 here: plugsocketmuseum "Not grounded twist lock socket and plug for mounting on a ceiling. A (lightweight) lamp or pendant connector plug can be attached to the cable. The plug has to be rotated 35º to detach (16). Images 17 and 18 show the components separately."

I note that you seem to have a hook for a potentially heavier pendant fixture beside it.