The security bolts became a standard fitment between October 1940 and February 1941. At that time, standard pressures for road use were all below 20 psi. A post-1940 WD motorcycle ought to have one for the looks, but perhaps best to simply grind the head on a Whitworth screw and fit it as a dummy.
Note that the SWD Norton Big 4 already had them as standard fitment from the beginning in 1938.
The sizes are according to the rim size, so WM1, WM2 and WM3.
Here some pictures, there were more makes I guess, and were made by the tyre manufacturers.
I made substitutes from countersunk 5/16" x 1 11/16" WW screws, with the appropriate chrome leather and steel washers and nuts. I only used brass nuts as I had a box full of them, so not necessary.
Hope this helps.
Cheers,
Lex
email (option): welbike@welbi**.net