Time to sum up where we are in this cause célèbre, I think.

The diameter of the centre pin in current production BNC plugs is
specified as between 1.32 and 1.37mm (0.052 and 0.054 inches). This is
the same for 75R connectors which are detailed in MIL-STD-348B (9Feb
2009), and 50R connectors, shown in MIL-STD-348 (20 April 1988). The
earlier document does not refer to the Zo, but the drawing is clearly of
a 50R socket with the extended dielectric surrounding the split pin.

Therefore - all compliant BNC connectors are mechanically compatible
between 50R and 75R from April 1988 onwards. There are NO mating damage
problems for current production items.

It has been suggested that prior to the introduction of IEC 169-8 in
1978, which defined interchangeability, there may have been differences
in the pin diameters between 50R and 75R plugs. This will only be
resolved by the actual measurement of plugs known to have been
manufactured prior to 1978 - or someone producing specification
documents from the time.

The definitive specification of BNC connectors would appear to be
contained within IEC standard document 169-8. This does not seem to be
easily available - copies must be purchased at considerable expense from
the IEC. Has anyone got access to a copy of this document ?