It was stupid o'clock after a baaad day and the answer seemed reasonable to me-----an old northerner with a Les Dawson sense of Humour. There is no specific reason for the holes, may just be a design carry over from an originally different method of fixing, may just have been an ink blot on the drawings, who knows. I have seen a tank packed up with rubber and a nut(Nylock) and bolt through that hole when the tabs on the frame had disappeared---made it a swine to remove and cost the customer a fortune in labour.
Thanks Hairy at least one person understands me, apart from the cat!!