This is one piece of used HP Agilent Keysight DC-26.5GHz 3.5mm (f) to 3.5mm (f) Panel Mount Bulkhead Connector Adapter. Part Number: 08513-20017 with washer and nut.

I tested 5 units with N5245A Network Analyzer, and 5 plots attached. up to 26.5GHz looks good. It seems can work up to 40GHz, but there is small area at 34GHz has little possible "mode" issue.