NB
With the non IGH OEM Brompton Hub, bike comes with a 12/16 cog set,
using the 54t crank.

The BWR comes with a 13/15 .. same spline, 50t the standard crank there.
BMX Shimano pattern I believe .. SA's S3X might be same..

If you got the Sachs/SRAM 3 speed , the 13/15t set on those is the AW3 spline/
Spares of those are handy , I use a 15t on my AW3/BSR.