HP used a special N connector called N Precision : the tolerances are tighter and have nothing to do with a common N connector. The limits on a N gauge are very special and, for example, a Weinschell N gauge has quite different limits on its dial comparing to a HP one (in some cases mating a Weinschell N connector with a precision HP will detroy one of the connectors). In the male HP precision the cylinder around the male pin is without the common 4 slots. Gauging the N connectors of all of the 848XX power heads gives the same value of recession of the central pin within 1/100 of an inch...
N precision is usable to 22 Ghz but above there is a waveguide effect about 23 GHz and it is used well beyond to 26.5 Ghz on the 8562X and 8563X SA with a degradation in flatness response (8566B can go to 24 Ghz but it is not specfied above 22 Ghz)