[Graham G3ZVT]

OldTechFan96,
I have never seen a brown one like that.

An older iteration of the MK plug, one without sleeved pins, was not so well designed.
I can't find an example to autopsy, so words will have to suffice.

The pins were secured to the plastic body with slotted nuts, then there were two plain washers between which the user was supposed to wrap the wire, then a backnut was tightened down.

The backnuts did have a slot, but unless the wires were thick, you couldn't easily use a normal slotted screwdriver to fully tighten them down because the top of the screw thread imposed itself in the slot.
In use, both the nuts on all three pins worked themselves loose.

So out of this dog's dinner of a plug the later MK plug that we all know and love was born, with its deceptively simple cord grip, captive securing screw, and those clever one piece terminal screws, with built in shake proofing. Truly a design icon.