I got an old Siemens RW666 LP player, but the cable is a little short... What plug is this, so I can get an extension cord?
That looks like a 180 degree 5 pin DIN they were fairly common a few decades ago. less so recently.
180 degree describes the angle populated with pins
It is a DIN connector for analog audio signals, that were initially standardized by Deutsches Institut fur Normung (DIN) . Some of these connectors have also been used in analog video applications and digital interfaces.
look at here for more info