That looks the best possibility yet.

There is that bent fitting on the back that could be either a fluid or electrical conduit.

There is also a small peg that must have been to orientate it.

Having picked some of the soil out of it I don’t think it’s a light fitting as you can now see that there is a second set of plug hole strainer shapes about 1cm behind the outer one, there also appears to be a deliberate gap in the ring surrounding it.

It could be some like that but it is a very heavy and solid object.

It was found here.

It certainly looks like a plug hole but the fitting on the back is too small for that.