Looks like ST 18/3 to me.
Electricians
Thanks ! You put me on the right track. I have found this socket that looks the exact same: BACHMANN Einbaustecker GST18/3 schwarz - Conrad.de
I still have to find a corresponding cable with the length I need.
Yeah just look out that connector is missing the locking clip. That can be important for something that's hidden away. On the other hand, the last one got loose anyways...
These cables and connectors are very common for under desk electrical wiring. So maybe look at office supplies shops for cheap deals on cables in different lengths. The connector on the other side can be harder to find, but a place like Conrad will have it for sure. Although I usually find the part on Conrad and then order on Ebay for half the price.
Edit:
I found this cable: https://www.conrad.de/de/p/bachmann-geraetezuleitung-geraetezuleitung-cee7-7-st18-schwarz-5-m-1839833.html
I can't find the female connector with the locking tab on the front. So I think the connector you found is fine.