NSH adopted saddle tanks in 1928, so this most likely is a 1925 to 1928 model. They began fitting JAP engines in 1925. This is possibly a 350cc JAP, rather than a 500.

NSH Motorcycles
Photographed at the 2008 Ibbenbüren Veteran Motorcycle Rally by Uwe Goedereis, friend of Craig Howell Licence: CC BY-NC-SA 2.0