The PM2424 has a full 4-digit nixie display. These are multiplexed, as can be seen by the occurrence of ghosting. The logic circuitry uses L-TTL and LS-TTL (Low-power and Low-power Schottky Transistor-Transistor Logic) ICs. Judging by the production dates on the ICs, it has been built in 1973 or 1974.
Two circuit boards contain all the electronics. This instrument looks a lot like the PM2423 so I presume it has been built in Japan, too.
I bought one of these in 2012. That one was in good condition. It had a tendency though to give erratic measurements every now and then. I suspect the range switching relays. In January 2013, I stumbled upon a second one, which was working just fine. Maybe it needs callibration, it seems to be reading slightly too high. But for now, I don't have the need to take this meter apart. When I'll do that, I can take some more pictures and do some more reverse engineering. In the meantime, these are nice assets for my lab.