Don't recall the puzzle either Eril, but you can confirm the unchecked second letter using a search facility such as this:
http://cfaj.freeshell.org/wordfinder/
Check the 'words matching' box and enter the possible letters for each cell within square brackets, using a full stop in position 2, eg [abcd].[efg][hij][klm][nopq]
The nature of these bell-ringing changes is such that any given note winds its way down the sequence in a sinuous string. In this particular example, G 's pattern is repeated exactly in the second block, creating an attractive symmetry in the grid - this is presumably why it was chosen to be highlighted. Does this sequence have a special name I wonder ?