This is where record cutters raise questions about cutting, and trade wisdom and experiment results. We love Scully, Neumann, Presto, & Rek-O-Kut lathes and Wilcox-Gay Recordios (among others). We are excited by the various modern pro and semi-pro systems, too, in production and development. We use strange, extinct disc-based dictation machines. And other stuff, too.
never seen before...
but maybe it is something like a phono preamp? "PU" for PickUp?
The 50/75 switch could be for oldschool DIN 50µs encoding or modern RIAA 75µs treble EQ.
Don't know what the knob does, more pictures could maybe clarify what these units do..