An update on the un-listed plate marking. I have recently seen a corner strip of four stamps bearing this plate marking under control X35 N40/4 Ochre - Brown. The control was in a different position and confirms it is from the Harrison Printings.
Further update on this plate marking. Shown is a control Q32 strip with identical plate marking confirming that initially it was used for Waterlow Printing Row 20/1. I have recorded Ochre - brown from 1931 through to the Harrison shades 1934 - 1935.
427 N43/3 Deep Olive - green. Showing Perf Type 4 variety from Plate 2c printing after re-chroming. The marking 20th left side got progressively smaller.