This was built for a Varnish Impregnation Unit. It could be Epoxy or some liquid used in Coil-Capacitor Vacuum Impregnation Process. This was fixed behind a Glass Window, when the liquid reaches this level, the circuit senses it and works like a limit detector. Hence the name Optical Proximity Switch. It is fairy simple in design. It worked after a few trials.
This was a custom design. We always should cost custom designs more, there is more time and materials used up. Only when some quantities are made, you can make it break even. Costing should also involve a R&D component, this is needed to build the in-house lab. Many good firms today, owe their present performance to the research effort and investment done by their predecessors.