Wing Palette do have a YP*D, serial PZ170 with undersides with black paint, with 100 Group...so, maybe somewhere mid 1944 onwards the Air Ministry Order was supplemented by another one?
http://wp.scn.ru/en/ww2/b/77/9/6#52
If memory does not fail me, some examples of 100 Group Mosquitos with black undersurfaces can be seen on references books (about the Mosquito). Sadly I do not have a single one at hand now.
NICK, do you know if there was another AMO about the machines of 100 Group (141, 157, 85, 96 Squadrons, etc??)
Adriano