Indeed a nice board, but there is something wrong with the schematics. Pin 6 is the negative IN, not the positive. So either, the pin number is wrong and the signal is using the inverting entrance or the pin numbers are right and the signal is goint to the non-inverting entrance. Seems to me the latter is the most logical.
You say it is a 'modified' circuit. Look spretty mucg identical to the circuit from the ZX manual, but maybe I am overlooking something