First off I am XXY, although I show none of the physical traits of KS, KS being a form of male hypogonadism, and as I have testosterone therapy I would be very disappointed if I did show any physical traits of KS.  My parents were very keen on taking photographs of their children, and I have official school photos of myself with other  boys too, and I would not have been found in childhood if the criteria here were applied then.  I was one of the shortest in my class, from 1961 to 1969, I was super skinny,  in my early teenage years I was suspected of having Anorexia.  

43 subjects studied is not enough frankly.  It's nice idea to find XXY boys who do not have KS in childhood, as KS only manifests  after the onset of puberty, which these researchers know, so why do they insist on saying KS is a genetic disorder when in fact it's the symtoms of a disease?    I may have had very small testes in childhood, but then all boys have very small testes in childhood how can any doctor be expect to note what would be tiny differences?

Physical traits won't find all XXY boys, nor will educational difficulty.  The absolute best way to find all XXY boys is a Barr Body test at birth.  You can still maintain your myth that XXY boys have Klinefelters' syndrome, no loss of pride there to admit you're wrong, and the boys get the assistance they need if they show they need assistance.  Then you can develop a truer model for finding  XXY boys in childhood, for countries that can afford Barr Body tests.