Follow-up question: Are the connectors standard chassis mounts? Or
are they some proprietary HP item? I would like to replace one, but
they are not listed in the manual as being one of the replaceable
parts.

Well, they probably suspect that some nut would overheat the precision
resistor inside, or move one of the "fudge" tabs that are in there to
provide a controlled capacity that allows them to rate the things to the
frequency they do. (Some are DC to 500 MHz and some are DC to 1 GHz.) A good
attenuator IS a precision piece of test equipment and should be well cared
for.

I've never replaced one, but the thread count is the same as some bulkhead
BNC's that I have, and would expect them to be replaceable if you took care
to NOT do at least one of the above. Look at the replacement connector and
ascertain that it is the same as the one you wish to replace before taking
that giant leap. That particularly pertains to LENGTH of the threaded
portion of the connector as well as the thread count itself. There's not a
lot of spare room on the inside. Just might require some machine shop
turning of available BNC connectors.

I have a few that were from canniballized test equipment. HP 312's, 313's
and old 8640's. some of those have SMA and SMB connectors that were swedged
into the cast frame and were immoveable without drilling out and retapping
for a larger connector. You might investigate what you have from that point
of view, since if they're already swedged BNC's, you can't install type
"N"'s as a one way replacement, there's not enough room.

W4ZCB