The Brompton chain tensioner keeps the chain taught (so that the chain doesn't fall off of the bike when folding), and acts as a derailleur on Bromptons with two rear sprockets.
Replacement chain tensioner assembly for Brompton bicycles without a derailleur (so your single, three or five speed Brompton). This can be easily identified by the silver (dirt may change the colour!) metal disc on the longer of the two arms.
Brompton standard chain tensioner idler (jockey wheel) set (pair) for Brompton bikes with no derailleur
Replacement chain tensioner assembly for Brompton bicycles with a derailleur (2 speed or 6 speed) This can be identified by the fact it has 2 identical looking black jockey wheels where the chain sits in a groove in the jockey wheel.
Brompton derailleur chain tensioner idler (jockey wheel) bearings / fixings set - pair - QCTIDLDRSET
Brompton chain tensioner nut for Sturmey Archer alloy shell hub This is the longer 19.5mm nut.
Brompton chain tensioner nut for SRAM / Sachs 3/6 speed hub (Note the hub internal is the same for 3 and 6 speed - the 6 speed has two sprockets and a longer driver to allow the fitting of them)
Brompton gear indicator / chain tensioner nut for Sturmey Archer steel shell hub (pre-2004) This is the shorter 16.9mm length nut
Replacement chain tensioner disc for Bromptons without a chain tensioner (1, 3 and 5 speed)
Derailleur chain tensioner idler (jockey wheel) spindle, single unit
Derailleur chain tensioner idler (jockey wheel) nuts/bolts, single set for Brompton
Replacement Brompton chain tensioner idler wheel for derailleur chain tensioner (CTADR). Discontinued in 2017