Those connectors look they made by LEMO, another company that produces excellent HV connectors.
ALDEN also make some nice convenient plug in HV connectors at good prices.
Dont worry about the voltage rating, if you feel unsafe just mount the connector on another piece of insulated material then onto your panel. You will then have a double insulated connector that is unlikely to flash over.
Its nice when you have a huge Hypot that you can test all these possible scenarios, its amazing what you learn about high voltage playing around with any Hypot.
I have managed to ID these and they are HV connectors from Hunting HiVolt testers rated to 30kV. My source had some new ones, and some factory pre assembled into Hunting HiVolt insulation tester extension leads. They should be more than happy at 5kV and as far as I know Teflon doesn't deteriorate as a dielectric with age. So good to go, and I am have FAR more confidence in these than the Millen HV plugs and sockets that have no proper cable retention on the plugs and minimal distance between insulation and the conductor where the cable enters the plug. I must say they look very iffy to me
Thanks for the help.