The Brompton Crankset 50T is a replacement set with left and right hand crank, including chainring and chainguard, as found on post 2013 Brompton Bikes.
 
A standard 3 or 6 speed Brompton comes with a 50 tooth front chain ring.
 
A standard 1 or 2 speed Brompton comes with a 54 tooth front chain ring - changing to 50 teeth will decrease the gearing by approx 7%.
 
The Bottom bracket is NOT included - If upgrading from the old fixed crankset then you will also need to change the bottom bracket (the old one was ISO standard and the new one is JIS standard).