Brompton uses a single large robust sealed bearing, Bikes ship with just the left,
but with a Rh threaded plug, a second one to balance the weight, can be bought..

L http://www.sjscycles.co.uk/brompton-...hand-prod2922/

R http://www.sjscycles.co.uk/brompton-...hand-prod2923/

note the new allen socket core axle - bolt, vs prior 24mm hex.
unscrew to swap out to Clipless pair made easy.