The PL-68 Plug is a microphone plug. It uses a smaller diameter than the standard 1/4" plug, so that it cannot be plugged into a headphone jack, which could destroy the microphone elements. It also prevents headphone from being plugged into a mic jack, which could result in confusion.
It has three contact points, instead of the common two.
The rear metal section of the plug has a small channel milled out, to allow the exiting wires to be tightly wrapped and tied by high-tensile string, keeping the wires from being pulled out or frayed during use. The wires themselves are attached using special hollow screws.