Found my Wx III ground school notes if it helps your manual Lee. One shows the duplex nature of the fantastic Louis Newmark Mk 30 FCS. Null indicators were used as part of the process to identify which channel of which half was suffering a problem during airborne fault diagnosis.
See attached samplers, please excuse the sketchiness, I was a different type of artist in my 20s !!!
I can photocopy and post you the whole book if required, theres about 35 pages of A4 and a selection of pilot's notes quizzes. pp