50 Ohm BNC Jack, Bulkhead Mount, 4 GHz
The 031-239: It comes as a kit with the connector body, a gasket, a lockwasher, and a nut — everything for a clean bulkhead install from the rear side. The center contact is silver-plated beryllium copper and brass, mated to a PTFE dielectric that holds the impedance steady across temperature.
The 4 GHz max frequency means this jack holds its 50 Ohm characteristic impedance across the DC-to-4 GHz band. The 0.2 dB insertion loss is low enough for most lab-grade signal paths.
Termination is solder cup on the center contact and a solder lug for the shield.
The 031-239 is listed as Active — no end-of-life notice, no last-time buy.