But anyway - back to 4721: with the help of the posse of gentle nudgers already thanked, I've cracked it. I was diverted for far too long by the decoy contiguous sequence of the shorter item that I can't believe wasn't planted very carefully by the devious Shark - just an extra demonstration of the genius involved in this construction - and I was wilfully staring at everything but the b***** obvious when following the advice in post #96!
Do check the novel on this very theme that I mentioned - a little Wiki and Google searching around the theme should find it - I've loved it - and other novels on parallel themes by the same author, since I was introduced to them six decades ago. A great read.
You too, Shark, if you haven't already, of course. Thanks so much for this great creation.