I have been in private correspondence with Andy about this already, and couldn't help a lot despite working on the area for years owing to my allergy to all things power operated plus a cunning plot to avoid actually working any of the so-called shunting frames in the area.
However, this is the picture I sent Andy of the inside of Carriage Shed North - this was taken when I was training Wembley Yard box in the 1990s (always made the best use of a walkabout!):Photo: John Hinson
However, looking into my various sketches of the layout, it appears the additional switches were added w/e/f 11th July 1965 when the new signalling on the main line was introduced. This controlled the access from the Up Slow into the Carriage Sidings and the 8 Reception Roads for Sudbury Hump. The switches were numbered 51-62 (signals) and 71-81 (points).
Some additional switches were added in 1983 in connection with a new carriage shed, the numbering was then 51-65 and 71-84.
However, when Wembley Yard opened some substantial changes would have been made to reach the situation in the photograph. Most of the area controlled by these switches was taken over by the new box, leaving only a few slots and the above-mentioned carriage shed.
Therefore, my initial thought that the switches weren't visible in the picture could be wrong - the box to the left of the diagram may well be the guilty party, with all but a few of the switches removed.
is tuned in at the moment, I'm sure we will get a definitive answer on this and probably on the other questions.