Cycle Monkey in Berkeley is a US Distributor/retailler , but I expect Nyce wheels in NYC can get them .
Mine came thru CM Wasson in California..
there is a lot of torque developed in Low range, but the special model for Bromptons transfers that torque
thru a lever with a Button that rests on top of the part behind the crank that carries the rear fold pivot ..
I needed no frame modifications on my Mk4 bike .. the BB is part of the whole crank assembly
as the shift is a shuttle pin passing thru the BB spindle center . center button is the shift from one side to the other .
conic rings center the BB assembly in the shell .. it screws into itself not the threads in the BB..
FWIW there is a 1.6x overdrive version, 'Speed Drive' , when fitted with a 34t chainring , in high range it acts as if you fitted a 54t chainring too.
It does take a chamfering of the edge of the BB shell*.. there is less torque in the overdrive against the frame , so the friction
of a gripper washer in aluminum between the frame and the crank is sufficient..
* not for the first time , Mavic had a BB assembly requiring the bike frame edge be conic rather than flat across the face.
Brompton already has a BB shell face that is not exactly flat, so the chamfering is not removing much metal.
and why the fitting of the MD really didn't require, any ..
Fietsbob, how did you get the third sprocket on the bike?
its not my bike , I just saw the way it looks .. the Brompton 2 speed is a hub with no internal gears
so there is a more hourglass shape to it . and a different way the driver is attached to what is a short driver freehub.
9 spline driver style is shared by the 2 and 6 (IGH 3) but not the BSR/AW3..
It can be done with 2 thinner spacers and Shimano Cogs .
there are custom adapters *, by the way to bolt on what is a braze on FD where the seat tube is .
it is placed behind the Seat tube a ways, since the seat post telescopes down behind the BB..
* Steve Parry in UK has some ,( a brompton modifier) I got one that the Wasson Shop got made ,
I Can resell, it .. , a Thumb shifter and a Bz on Ft Derailleur .. makes it work .