Here we have a listing for the hard to find fine pitch serrated A2 Stainless flange nuts.  

They are M10 x 1.25 pitch. 15mm A/F (socket size).

Please use some anti seize lubricant to ensure they do not seize up in the future.

I have had these made.  They are very good quality. Various uses on your bike (sprockets etc)

Thanks for looking!