The type of small plug often used with aerial and earth sockets on mains powered radios, years ago. Many other uses, too. Speakers etc.
A split pin with (usually) red or black plastic cap that screwed on, trapping the wire. Predecessor to the modern banana plug, and about that size, but probably a tad less than 4mm. Probably Imperial, not metric.
Yes, I've Googled, searched eBay, Rapid and Maplin, but must be using the wrong description. There must be someone out there still selling them