Early T150 A75 4 speed gearbox sprocket locknut . To stop the gearbox leaking oil down the mainshaft high gear, Use with 1 x 57-1956  oil seal  and 1 x 60-4374  O ring. Must use the later clutch hub 57-2580 with the recess. If using the earlier clutch hub, delete the O ring or modify the hub to take the O ring