Remembering the back story of the HU5 it is entirely possible that the unpopylated instrument panel shares a part number as far back even as the S58 from which the HAS1, HAS 3 and others were derived. If so the holes may have been populated on earlier marks, but not needed on the Mk5. It would be cheaper, and better logistics in service, to produce common panels and populate the unused holes with blanking plates at aircraft production. A look at HAS 1, 3 and 5 panel part numbers in the Topic 3's might be illuminating. Similarly, if the hole fillers were wired there will be details in the wiring diagrams, Topic 10.
Thus opening 20 might indeed have been a cable out indicator on the 1 and/ or the 3 but was not needed for the 5, so was always fitted with a blank.