At a first look this clampmeter looks like a very old Bakelite instrument but if you push the button under my thumb the display lights up. It is probably from the 80’s (the pdf manual I found is dated 1985) and made in the USA.
It is rather heavy, the shells are rigid and the screws go in metal threaded standoffs. I could not find a schematic, only a one page user leaflet. So I do not know how to adjust it.
It is an 0-1000Aac current probe that has two modes. Continue and Peak. The latter is for measuring inrush current. But it also measures 0-1000Vac using two probes that I do not have. They go in the bottom with some strange bajonet connectors. It is also a 0-1000 ohm-meter. You need a special probe for that too. I do not have the probe and I do not know how it looks. You plug it in the sideconnector that looks like an 3,5mm audio/headphones socket.