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They could just be regular 3.5mm mono plugs but the tips came off. I recall that some cheaper and older plugs have a similar construction to that where the tip would be screwed on tightly rather than a solid tip on some more higher end plugs.
Metal area is approx 1/2" and the tips have screw threads. It’s a pair of 1971 SONY headphones and I believe these were modded with these connectors.
Back in the old days the connectors unscrewed at the ends, and that is what you have here.