It is a DIN connector, commonly used for all connections to amplifiers, remote cabled signal sources, ie turntables, tuners, pre-amplifiers, etc basically used with multicore screened cables carrying low level signals in the audio range. Also some control signals for remote CD players in the car audio scenario. Later RCA coaxial connectors were used for individual audio or video channels. This connector hails from the 1979-90s you need to be able to solder well.
There were many versions of DIN connector with more or less pins Pins 1,2 and 3 were the original left and right audio input with pin 2 as common earth. (Pin 2 is also often connected to the housing screen)
These may still be available from RS type shops like here: https://uk.rs-online.com/web/p/din-connectors/0463394/ Though you may need the non Pro version without the locking ring, depends upon your socket. Some types have a press in latch/lock too. Just search on DIN connector. (In case it is of interested DIN stands for Deutsche Industrie Norm) the German equivalent of BS) they were the standards adopted by most of Europe for the audio and automotive industries) I was in the automotive side in Car audio many many years ago)
Note there is an established use for each pin, but it does not matter in your case, be aware of whether you are looking at the front or back of plug or socket when wiring it up. I suggest a look at WikiPedia: https://en.wikipedia.org/wiki/DIN_connector this has all types.