Thanks for your thoughts. I know the shape is slightly different but I cannot see that being the reason for the name. The lack of replies here suggests that no one else really knows.
Your comment about the position made me think though

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The position of the G for both types is very variable and cannot be used as a guide BUT perhaps the first sheets of the "Flying G" found had the G in a higher position against the normals the finders were comparing it to and, not realising how variable the positions were, thought that this was a constant feature hence the name.
AQ