so if it's a PTE 100 it will need an old philip harris digicounter probably.I believe it is the old and discontinued PTE 100 connector.
Rarely they may turn up on Ebay. A regular BNC plug and socket can be used instead but they are only rated to 500v so you are likely to be pushing it a little. A better option is to fit the SHV BNC which is rated to 10000v but there may not be enough room inside the housing to fit the longer socket.
Do a search on SHV BNC and you will see the size issue.