The fin head dimension (from the original Larry Tuttle (Water Rat) drawing is 0.620" (+0.000 -0.004)
so the tolerance range is 0.6160"-0.6200" which according to my inch/metric conversion program
give the metric dimensions as 15.646mm - 15.748 mm.
So if the fin head width is 15.748 mm max and it is supposed to be an nice snug "push fit" I would guess
that the fin box width would ideally be about 15.75 mm.
Hope this helps,