Here we have a listing for M12 x 1.25 pitch A2 stainless flange nuts. Suitable for sprockets.

They take a 17mm spanner.  The diameter of the flange is 23mm.  They are smooth faced.

Please use some anti-seize compound when fitting (to ensure they do not seize up in the future).

Free postage in the UK.  Free Returns. Thanks for looking!