This beautiful HP counter does not goes high in frequency compared to modern counters. It is only 55 MHz. But nixie display equipped instruments are awesome. I had two. The other I gave to a friend who loves nixies too. This counter has the voltage meter option. A bit unusual and I have no clue why they offered this option. The counter has an input for an external reference and two marker inputs. There is a HP publication that talks about a modified 5326 as part of a HP8407 VNA configuration. I have that VNA too but could not find more info on the mod.
Mine also has the extra digit option. Both needed some TLC and repair. My friend made some pictures from the pcb’s to ease the repair (finding component locations)
The pictures are also in a pdf.
HP 5326B Timer Counter DVM Operating and Service Manual.pdf (255MB!, choose right-click | save-as)