A stainless nut for adjusting end float on the spindles for Lightweight Webbs. This is the long nut with the external thread for the fork damper to screw onto. 
 
Overall length: 1.125"
Internal thread: 7/16 x 26TPI BSCy
External thread: 3/4 x 26TPI BSCy
 
These forks where fitted to 1930's Velocette MAC and MOV models. Possibly others, so if you know of any other bikes that use the same forks we would be interested to hear from you.