Brompton Chain tensioner / gear indicator nut:

(This is the nut with a hole in the side)
Later hubs 2003 onwards (normally with alloy shell) use this nut.
The difference between this and the pre-2003 steel shell nut may be small - but it is essential to use the right one. 

Brompton 2020 Q part code: Q101145
Brompton Q part code: QCTN3-SA[2]
Old Brompton Q part code: QCTN3-SA

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