Sturmey Archer clutch spring
The attachments show you where this spring fits
Used on the following sturmey Archer hub (and probably more):
Brompton Sturmey Archer BSR - standard ratio 3 speed
Brompton Sturmey Archer BWR - wide ratio 3 speed rear wheel - as used on 6 speed Brompton folding bikes
Sturmey Archer ref: HSA128
Made in Taiwan