This spring is the return spring fitted to the rear brake arm on pressed steel brake plates (almost universally fitted on Nortons from the 1930's to the 1960's.
This item is the same as 0387, except that it has been polished on its outside and chromed (good quality UK chroming with copper plate under the chrome)
As with the springs above, a good quality original was used as a pattern.
Note: This particular spring has a slight kink on the end that fits on the brake arm - this was the spring type fitted to outwardly kinking rear brake arms. However, it is relatively simple (particularly with a bit of local heat) to remove the kink, so it can be used with the straight type arm, also commonly fitted.