I have EA 50's and have never noticed the empty groove in the back (I run them on a bike with gears/brakes). But then I use thick cinelli cork to wrap and that might be the difference over a thinner tape.
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Bikes: 1996 Eddy Merckx Titanium EX, 1989/90 Colnago Super(issimo?) Piu(?), 1990 Concorde Aquila(hit by car while riding), others in build queue "when I get the time"