Originally Posted by Jipe

Its just one option.

There are several brands/vendors selling similar things.

And indeed, different versions for T-line, P-line and also for C-line and previous years Brompton.

I am now always skeptical about Chinese third party parts for Brompton, I had so many disillusion with parts that do not work or/and are not reliable, even from expensive Taiwanese brands.

I prefer always standard parts from well known brands if possible like installing a real cassette with a real rear derailleur from Shimano, SRAM, Campagnolo... but it requires moving to a standard rear OLD and to find a solution for chain tensionning (as shown by other forum members, unfortunately, the excellent Birdy III solution cannot be mounted on the Brompton due to its special bottom bracket box).

Its really a pity that Brompton choose this poor 4s system instead of installing a real modern rear derailleur+cassette.