The feedback capacitor can be a value between 80 and 160 pF. (= C 10)
This is the HF feedback (de-emphasis) capacitor.
By changing the value, you can 'tailor' the HF frequency response.
When working correctly, the amplifier in the U67 mic rolls off the highs (down 10dB at 15kHz)
This compensates for the high end boost of k67 style capsules.
You can sweep the amplifier response to check that it is correct. Info is in the U67 build thread.
Negative feedback from the transformer is used for this, so it is a common mistake.