These are made to go in a modified 1.125" steel steerer tube, offers a quick height adjustment.
http://www.sjscycles.co.uk/satori-ha...ser-prod23752/

one needs a shop to behead the Brompton riser, and accurately weld on the equivalent
of a threadless fork steerer tube of steel, that gets slotted to be compressed in the clamp
of this thing.
.. then takes a common threadless stem to fit your bars