This is the correct type 'volute' spring of the type originally used Norton Internationals (and also OHV and SV Nortons) to hold the exhaust valve lifter cable in tension correctly. It it fitted between the Exhaust lifter lever and the adjustable cable stop on the cambox backplate This particular spring is took some searching as they are now very uncommon - but is of both a similar length and strength of the original. You should note that CS1 cambox's are identifiable as they have there location for the cable adjuster cast onto the front cambox leg, rather than the cambox backplate, therefore on an Inter/Manx the exhaust lifter arm points upward, with spring between arm and peg on the backplate, while on the CS1 the arm points downward with spring between arm and cambox leg. These springs were also used on pushrod and sidevalve engines, although with a different exhaust lifter mechanism 28/11/11: Incidentally, watch out for future updates. Due to requests from numerous customers I am having a batch of the eccentric hardened pins made, as fitted to SOHC cambox's with the exhaust valve lifter boss on the cambox backplate casting. I am also having the castings made for the lever that fitted onto the eccentric pin. Expect availability of both in the next 3 months.