For the gangwayed passenger stock - i.e. not including the catering vehicles there were two basic layouts.
The first layout - i.e. the one that was applied to most vehicles, was that on both sides (the dynamo and the regulator sides, the battery box was (when viewed from the side) to the right of the centre line of the underframe. On the dynamo side, the nearest edge of the battery box (not the frame supporting it) was 2'-1 1/2" from the vehicle c.l. and on the opposite (the Regulator) side this dimension was 2'-3 1/2". Note this layout did not only apply to TSO's, FO's. CK's, SK's etc. but also to the BCK & BSO
The second layout- the one used to avoid the battery box being behind the lower stepboards, moved only the dynamo side battery box to the left of the underframe centre line. The the nearest edge of the battery box on that side only, being 10 3/4" to the left - and as before, when viewed from that side. This layout applied on the gangwayed passenger stock to the BSK & BFK
Why isn't it mentioned in Parkin? I've no idea - the sketches were not however done by the author. It wasn't as a result in a subsequent change or modification during construction, these postitions were laid down from the start, the earliest drawing I have dates from September 1949......
Bob.
p.s. Any lining up with bodylights is entirely co-incidental!