Here is the transfer function and cutoff frequency (pole) of the first stage low pass circuit of the Pentek whitening board.
R1 = R2 = 49.9 Ohm, R3 = 50 kOhm, C = 0.01uF. Given a differential voltage of 30 volts, the voltage across the 50k resistor should be 29.93 volts.
So low pass RC filter with one pole at 1 MHz.
I have updated the schematic, up to the changes mentioned by Rana plus some notes, see the DCC link here: [PLACEHOLDER]
I should have done this by hand...