12 Signal Group, the TA element that was designed to form a line of communication from the channel ports/airports into 1 (BR) Corps, terminating into 16 and 22 Signal Regt dets/commcens, back when we practicing the fallback to the Rhine!
Means that your beast, if used for radio-relay was fitted with SR C41/R222. With the "books" stencil, may have been used for repair or radcon.
12 Signal Group consisted of (back then), 32, 34 and 36 Signal Regt (V). I think that 40 (Ulster) and 38 (Scottish) Sig Regt (V) may have been tied in, but they had different roles.
The lockers weren't usually marked, this would have been a local thing. The other stencils are also non-standard, thus local/specific to the unit.