Hi David and Micke,
I asked my friend, who collects 15th-16th century crossbows and all sorts of accouterments concerning a possible standardization of the width of the recess in the center of the nut that we had been discussing.
He told me that his collection comprised 6 (!) crossbows with composite horn bows, 19 cranequins and some 200 quarrels/crossbow bolts from various provenances, including some incredibly rare incendiary arrows.
His earliest crossbow is the fine and perfectly preserved piece from the Harold L. Peterson colln. (attached here), and it can be dated as early as ca. 1430-40!!!
Interestingly, he thinks that there was no such thing like a standardized space between the nut 'fingers'. He also emphasized that a great number of the bolt shafts in his collection had oval (!) rear ends while the rest was obviously cut to shape to fit that space between the nut 'fingers'. His conclusion is that the quarrel shafts were only cut right before they were about to 'see service'.
And: not all quarrels fit the nuts of any crossbow.