You can still get a very good idea as to comparative overall circulation, eg: hair detail, extent of wear, rubbing of lettering etc, even if the actual coins you are looking to grade, don't appear in Derek's book. The basic principles are the same with all coins.
Alright I guess it's the picture (or rather the lighting) - it looked different enough to me but looking at https://www.pcgs.com/valueview/george-v-1911-1936/1931-1d-s-4055-bn/3948?sn=203068&h=pop and https://www.pcgs.com/valueview/george-v-1911-1936/1931-1d-s-4055-rb/3948?sn=203069&h=pop which seem to have the same generally sharper design show that the feathers are the same.