Having had a look in my (junk)box with N-connectors I found that most
silver plated (professional) units have 50 Ohms stamped into the body
,whereas the nickel plated units either have no identification or a
specific manufacturer's code .
The Farnell catalogue you referred says these N-connectors are nickel
plated . Try to find any stamped code on your units . If any I'll be
happy to compare with those on my connectors.

Frank GM0CSZ / KN6WH
==========================================
Since posting earlier I've been on to the actual manufacturers and they
reckon these plugs are actually 50 ohms. So the supplier's insist
they're 75 and the makers say they're 50! I need to know for sure, as
one or the other is in error. How can one tell, by visual inspection
and or physical measurement, which type is which? There must be
something that's visibly different about the two types.
THanks.