[MrFixr55]

+1 on above comments. It was very common for radio manufacturers to put a non-standard jack on the rear apron of the chassis for power to the phono turntable. Sometimes it was 3 pin so the record changer's switch would power on / power off the receiver chassis. In this case, the power switch was not usually on the volume control off and was labeled Off/Phono or On.

However, chances are that this chassis was available as part of a tabletop radio / phono combo where the turntable was not a changer. Per the schematic for your radio, both legs of the the "phono power socket" were switched; one by the Radio power switch, and the other by a "Radio - Phono" switch. Verify that on your set, the switch exists and can be switched to Phono. It may be hidden or its travel blocked, as this is an AM / SW Radio. This is a transformer powered radio and should be a real good performer for a table radio.