Summing fader module Studer 980 mixer in Calrec S2 sidecar.

[Matt Syson]

If this piece of the circuit is being fed from an unbalanced source then the input pin indicated SHOULD be grounded but won't actually make much difference in real life. Abbey Road may be thinking this input terminal was being fed with signal in which case the pin P1-4 must be grounded to retain unity gain operation (basic differential input configuration)
Matt S

 

[abbey road d enfer]

If this piece of the circuit is being fed from an unbalanced source then the input pin indicated SHOULD be grounded but won't actually make much difference in real life. Abbey Road may be thinking this input terminal was being fed with signal in which case the pin P1-4 must be grounded to retain unity gain operation (basic differential input configuration)
Matt S

Agreed. Poor definition made me think it could be the inverting input there.