Differentiating between 3/16 Whiworth (now 1/4 BS) at 0.445 inch Across Flats versus 7/16 fractional (0.438 inch) can be difficult with rusty and abused hardware. But the absence of a 0.445 spanner in the above list suggests that they preferred a fractional hexagonal bar stock for that particular size over Whitworth.
So interesting!
It happens that there are people who have sorted original vintage Douglas nuts into zip-loc bags.
If we take 5/16-25 nuts - and therefore certainly Douglas - as an example, I can report:
1. There are 5/16-25 nuts that meet the BS standard: 0.525" across flats, and 0.250" thick. (Actually the thickness is slightly more than the standard , most falling into 0.255 +/- 0.005".) They are plain nuts, with no machined-in washer.
2. There are also 5/16-25 reduced-hexagon nuts. Looking at very original nuts, not rusted or ruined and maybe with original nickel plating, there are TWO sizes. Some (around 60% in my bag) are definitely BS hexagon (0.445 across flats), and are much longer than the Standard at mostly 0.345 +/- 0.005". The other 40% are definitely 0.4375 (7/16)" across flats and around 0.310 +/- 0.010" long. Both sets of nuts look like old, original, Douglas nuts!
I'm happy to agree with Doug's DT spanner references and say that some Douglas nuts were made from 7/16 bar stock, even if some others were Whitworth.
It's quite hard (but possible) these days to source imperial hex, and almost impossible to find BS hex. 7/16 sounds close enough.
Cheers
Leon