Hi All:
Well, I have some old gear using Lemo 304 bulkhead connectors which are enormously expensive, about 10x what you'd expect. I want to replace them.
The bulkhead connector contains 4 pins and a ground shield. It fits thru a 6 to 7mm diameter hole and has a round 9mm dia fascia/bezel to allow the 12mm nut on the back to run up the 1/4" or 6mm barrel from the back of the bulkhead and secure the connector.