Sturmey Archer clutch spring cap
A very small part of the internals of the Sturmey Archer hub gear (you probably lost it, which is why you are looking here!)
As used in the following (almost certainly incomplete) of Sturmey Archer hub gears:
S3X
S-RF3 / AW three speed hub
BSR - Brompton Sturmey Archer BSR - three speed standard ratio hub
Click here to see its location in the S3X s-speed sturmey archer hub
Sturmey Archer ref: HSA129