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 is normally 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 slightly different exhaust lifter mechanism - but the normal means of adjustment was the same, and these volute springs work much better than a standard coil spring and are the correct fitment for this application (see also Item 0865 for OHV style arms)
Fitting Notes -
it is normally not possible to purchase cables with the correct spring and adjuster fitted, as per the instruction sheet shown in this listing (see second and third photograph) - as the volute spring and cable adjuster normally need to be fed along the inner cable before a nipple is soldered in place.
Normally, I will buy a standard air/magneto cable (and ensure it is has a cable adjuster with it, with the same thread and diameter as the hole in the peg (SOHC Cambox) or threaded casting top (OHV Norton rockerboxs), and then remove nipple, fit volute spring and adjuster as per diagram, and then re-solder nipple (and splaying cable as part of resoldering), to ensure the adjustement is just right. It is not as difficult as it looks, and result in a well adjusted ex lifter. The exhaust lifter cam should not be touching the rocker arm in normal running, the spring just holding the arm off touching, but as soon as the decompression lever is pressed, it will touch the rocker arm.
Update, July 2021: We may shortly offer these Decomp cables with volute spring and adjuster already made up - email for details on email@example.com