This is the port that was on PCs (not Macs) to plug in keyboards and mice, before USB or wireless. 

I’m actually even older than these ports, but figured it’d be more recognizable. 😅 

 Here’s the Apple Macintosh ports of that era. https://image.nostr.build/6dba8e19aef51e764faaea90ebbe222b4d20d6ff67578b6ea9b5fe8383494c24.jpg