Many years (and brain cells) ago I recall looking at an old L-13
spar end cap and it had a very large number of bolts going through
it. It seemed like it was very close to what I remember as a hole
to edge and hole to hole distance requirement. My calibrated
thumb nail showed it to be within what I remember as a 2D
distance but I remember that from rivet holes to edge on wing
skins. I wonder if some of this is caused by hole to hole to edge or
radius cracks and if so that would be a very serious concern. Also,
I believe that the 2D used to be 1.5D and there had been questions
if that was hole edge to edge or hole center to edge.