This is a fairly basic 4 channel mixer with built-in spring reverb. I don’t know what happened to Pre-mixer 1,2 or 3 but I would assume this was the version that made it to production.
This mixer appears in the 1975/6 version of the Bell Musical Instruments catalogue so that gives us a rough date for manufacture.
Single high impedance input on each channel with controls for volume, tone and reverb. There is a single mono output plus a stereo in/out socket for external effects. There are also master volume and reverb level controls. The mixer runs off 240 volt using its built in power supply.