I believe COMP mode displays just black on the roller blind, and only operates the compass ring, which is active in all modes, see Fig 4 in the aircrew manual (second link below).
I have been given a copy of AP3456D, Part 2, Chapter 3, AL14, May 1969. This short chapter is just a rough overview of the instrumentation, which is nice, but lacks the detail.
See:
https://scottbouch.com/mcfs/lightnin...ent-system.pdf
There is more information about the Nav display operation in the Aircrew Manual though:
See:
https://scottbouch.com/mcfs/lightnin...nstruments.pdf
And a little in the Vol1 on TACAN:
See:
https://scottbouch.com/mcfs/lightnin...ov64-tacan.pdf
You'll see though that these documents provide good descriptions, but little about how to use the kit, hence asking for more help from pilots!
Cheers, Scott