If I remember correctly some systems from that era tried to get away from individual AC power for every piece of gear. If there is no AC cord on the TT that's what you have. That strange plug was inserted into the receiver to provide power for the TT.

This sounds like a turntable that originally came with a rack system. As stated, it's power supply voltage was supplied by the main amplifier. You'll have to be pretty handy to get this table up and running. I'm going to guess you're not that handy if you're uncertain what the plug type is.

My suggestion would be to scrap this table as its retail worth is possibly in the single digits. Check the classifieds for a second hand table that fits your budget or splurge for a new table that will give you all the benefits of the last twenty years of turntable design.