Mine is on the bench at the moment so I measured it . . Its small!
Thread appears to be 40 tpi (my thread gauge is not very precise at this end of the scale).
Thread diameter = 3.50mm / 0.1375 inch
Total length of bolt shank = 14.58mm / 0.574 inch.
Thread length = 7.27mm / 0.286 inch.
Plain shank length = 7.26mm / 0.286 inch.
Plain shank diameter = 2.74mm / 0.107 inch.
Hex head diameter = 6.32mm / 0.2485 inch.
I thought it was BA 4 - but it isn't !
1/8" Whitworth is 40 tpi, but 1/8" = 0.125" - not 0.1375" !
It looks like a "Special" . . . .
Time to get the lathe fired up
My avatar shows the late Len Rich in 1970 with the bike I now have - a 1958 Ariel VH