It's a Latch, YOU CAN buy the parts Kit and change that .. I did. QRFCLIP-RETKIT check an AMS Brompton Dealer or ..

http://www.sjscycles.co.uk/brompton-...14659/?geoc=us

Brompton makes things retrofitable back to earlier Models. [ Maybe not the original series but certainly the 2nd one , like Yours]

reaching down behind the seat tube, you touch a lever opening the latch . Rubber suspension Block is retained (if you want to)