Imperial Spanner Combination Set - 6 Piece Whitworth

For some reason now lost in obscurity,
the distance across the flats of Whitworth hex nuts and bolt heads
is not in fractions of an inch and follows no discernible pattern.
The sizes stamped on Whitworth wrenches do not refer to,
the distance between the flats, but to the thread diameters.
Sizes 1/8 inch
3/16 inch
1/4 inch
5/16 inch
3/8 inch
7/16 inch