The new Brompton "Spider" crankset was introduced on new Brompton bicycles at the beginning of 2013.
This is a replacement for 44/50/54 tooth chain ring complete with guard.
It can be used to replace the 44, 50 or 54 teeth chain rings on a "spider" type crank. (a different chain length will be required if you change the chainring size)
A standard 3 or 6 speed Brompton comes with this 50 tooth front chain ring
Brompton Q part code: QCHRINGA-SPI-44/50/54