It looks from some info on EEVblog the PM6306 could be a rebadged Wayne Kerr 4270 which might explain why Fluke are struggling to find information. This seems very similar to the situation I found when getting info on Pendulum Frequency counter, which is the same as the Fluke PM6690, so they certainly do rebadge things.
Some info here
https://www.testequipmenthq.com/data...-Datasheet.pdf