So Colin Cowton at Drawmer UK got back to me right away with the following info, preserved here for posterity:
AUDIO CALIBRATION
The audio calibration of the 1968 should normally not need any adjustment. However if component changes have been made, it may be necessary to adjust one or more of the pre-set potentiometers. To adjust the presets you will require an adjustable level, balanced test signal of 1KHz sine wave, initially set to a line level of +4dBu (1.23V RMS. You will also require an accurate VU measuring meter (with adjustable gain), an oscilloscope and a sweep scope.
The front panel of the unit must be initially adjusted for both channels, as follows:-
Threshold rotated Counter Clockwise to 'OFF'.
BIG switch to the 'Off' position. (Up).
Attack set to position ‘1'.
Release set to position ‘1'.
Gain rotated fully Counter Clockwise to '−20dB'.
Bypass / Normal / Side Chain initially set to 'Bypass'. (Right).
Stereo Link switch in the 'Off' position. (Up),
Meter set to VU position. (Middle).
The following series of tests will ensure all the features of the 1968 are operating correctly. In most cases, the conditions for each test should be continued into the following test. Consequently, it is suggested that all test should be carried out in the numbered sequence.
1: Turn the −3 VCA Presets R46 and R78 fully counter-clockwise.
3. Insert a sine wave signal of 1KHz into both 'LINE' input XLRs at a level of 0dB (ref +4dB) and allow the unit to warm up for about 15 minutes.
5: SET VU METER.
With the unit in Bypass adjust preset R14 (channel 1) and R190 (channel 2) until the meters indicate 0dB on the VU meter.
6: V.C.A. CALIBRATION
6.1 Connect an audio level meter to the output of channel 1.
6.2 Switch the unit into ‘Norm’.
6.3 Rotate the Output Gain pot to 0dB on front panel graticule.
6.4 Adjust presets R172(CH1) and R173(CH2) until the output level is set to +3dBu for each channel..
6.5 Adjust preset R46 on Channel 1 to give a gain reduction of 3dB. The VU meter should now read 0dB.
6.6 Repeat for Channel 2 (Preset R78).
6.7 Rotate the THRESHOLD control on Channel 1 fully clockwise to apply 20dBs of gain reduction to the unit.
6.8 Adjust preset R37 to give a gain reduction of 20dB. The external test meter should also read 20dB gain reduction.
6.9 Repeat for Channel 2 (Preset R69).
6.10 Switch the Stereo Link switch ON. Check that Channel 1 and Channel 2 track within 0.5dB of each across the full Threshold range.
Switch the Stereo link OFF.
7: G.R. METER CALIBRATION
7.1 Switch the meters to G.R.
7.2 Using Preset R50 set the GR meter to give a reading of +3dBu.
7.3 Repeat for Channel 2 (Preset R82).
7.4 Adjust preset R48 on Channel 1 to set the GR meter to read 0dB.
7.5 Repeat for Channel 2 (Preset R80).
7.4 Rotate the THRESHOLD control on Channel 1 clockwise to show 10dBs of gain reduction on the VU meter of the unit.
7.5 Switch the unit back to GR and adjust preset R36 to give a gain reduction of 10dB.
7.6 Repeat for Channel 2 (Preset R86).
7.7 Remove the Gain Reduction. Repeat above steps until no further adjustment is necessary.
7.8 Switch between VU and GR and check that the meters are identical. Adjust as necessary.