Panel-mounting double-sided DIN1.0/2.3 connectors (also known as 'back-to-back' or 'bulkhead through-adapter'). They have gold-plated, beryllium-copper, centre contacts for optimum mechanical performance. Connector bodies are gold plated, are insulated from the panel, and have a nickel plated mounting bush.
Panel hole diameter 8.2mm. Body has anti-rotation flat, 7.2mm across body at flat. Suitable castle nut spanner, with 6.35mm hex drive connection, is 55-8195.