Headphones.
I do not have the exact headphones for the BC 221 frequency meter so I selected a set made in America.
I believe they should be P18 or P20 style, but I do not know what they look like.
The rubber wires have perished and I think unable to be repaired.
I had seen three wire mains power cloth covered cabling on eBay, so I ordered some.
I was able to unwind one wire from the twist.
I selected a reasonable length and fashioned a new lead.
I bound the ends and junction with black industrial sewing thread.
Looks almost as good as new, anyway they work.