Old building unknown socket - high voltage for stove?

Instead of the OP, I would first be very curious about what kind of nailed antenna cable is running to the power outlets :-(

This is a strap line. They are still nailed today.

If this is the only supply line to this double socket, however, the Terkodose should not have any special significance.

There is another black cable on the right, which goes to the Terko.

True. So much is missed on such a smartphone display...

Many thanks for your answers, Yes, there is indeed a thicker extra cable running to the mentioned Terko socket. What was such a "non-polarizable" socket used for, or what could I need something like that for? I assume I will have to renew the cables anyway.

What was such a "non-polarizable" socket used for, or what could I need something like this for?

For everything in the low voltage range, as mycraft described. It could have been anything. A specially protected circuit, a voltage different from the normal power grid, e.g. 110V network, or a DC supply. Something where it was important to ensure that only the devices intended for it could be connected.

As said, it could not have been anything related to high voltage or the like, so the purpose is presumably no longer relevant. If you are more interested, you would have to have an electrician investigate what kind of cable it is and what runs through it.