The PM2421 is a digital multimeter. The case is apparently intended to be placed on a shelf, although it does have a tilt support for bench use. Its display has 4 nixies. The digital circuitry is based upon TTL.
I bought this meter in spring 2001 on the Veron radio amateur flea market. The person who sold it, told me it had been used at a school. No documentation came with it, and I haven't found any yet.
When I tried the meter, it gave an overload indication and it behaved somewhat strange. When I switched ranges and flipped the switches on the back, it sometimes gave a 0 readout, as it should when no input signal is connected. Apparently, something is wrong with the switches or with the autoranging circuitry. But as I did't have any documentation, I balked at the idea of analysing the circuits.