Probable use is to precision adjust flow rates to exactly control travel times of hydraulic cylinders. Adjustment precision and repeatability is better than one part in 500.
Rexroth R978901723 LFA16H471/FV/12 7871 Made in USA. Basically, you have an attachment that fits a 2-1/2 inch square manifold or valve or regulator or whatever. There is an input 3/8 hydraulic fitting. As you adjust a large ten turn knob, numbers change from 0 to 10 in a mechanical readout and a quarter inch hardened steel pin extends downward for low numbers and upward for high.
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