The "PSV" line of SWR meters was produced by the former Svazarm's company Radiotechnika at the end of 70's and the begin of the 80's. The name of the line is derived from Czech name for "standing wave ratio", "Poměr Stojatých Vln". Inside the very rugged sheet metal box is a very stable directional coupler, based on thick sheet metal groove and three conductors inside it (main and two couplings). All made from silvered steel, very stable. There are PL239 connectors used, sic!
The circuit of the SWR meter is traditional, no specialities. Because of the size of the directional line, the meter reading varies at a lowest band, 3,5MHz, and because of it there is a specific SWR scale at the meter of PSV II. From 7MHz up, the directional line is very precise and no individual scale calibration is needed up to 145MHz. The typical transmitter power is up to 100W, at VHF a successful reading is possible with about 10W only.
The manufacturer's documentation is available.