Sturmey Archer "blue" 5 speed gear indicator chain
If you have a 5 speed Sturmey Archer hub gear on your Brompton and need to replace the indicator / gear change chain - then it's best to clean it and check the end (see picture).
It will be marked blue (this one)
You can also check on the hub itself - where Sturmey Archer will have stamped the manufacture date - just to double check.
This was Brompton reference: QGICH5B
Brompton part number: 9011124
Sturmey Archer part reference: HSA317