This is the castellated nut as fitted to the front fork spindles on early Douglas models - approximately 1912-1919. They are CNC manufactured by us, copying an original Douglas fork spindle nut as a template.
The nuts are deeper than commercially available nuts, and use the the correct imperial 0.525" accross flats hex, to fit a 1/4 Whit spanner, as per the originals.
Note that the thread on these nuts is 5/16 x 26tpi BSC (not Douglas 25tpi thread). I am not entirely sure, but believe original early Douglas fork spindles may have used a Douglas specific 25tpi thread. Although our nuts will start to go on an original fork spindle with a 25tpi thread, it is not good practice and I cannot advise doing it. Therefore - if you are unsure, please try a commercially available nut 5/16 x 26tpi BSC nut on your spindle first (or use a thread gauge if you have one), as unfortunately we cannot accept returns on these nuts (as we cannot check the thread will have been stretched) - hope you understand.
Apologies these nuts are not cheap - but they are not easy to manufacture, each slot has to be individually machined which makes them very time consuming on our machine (and actually when I tried to speed this process up, ended up snapping the miniscule endmills!)
Each nut is linished and comes with a stainless steep split pin