I swapped the 2013 OEM Firm suspension for a green Birdy one(hardest model/color).
I got mine from the local dealer, BFold of New York City. It was US$20? Can't remember
exactly, did the mod back in 2013. Weight is almost the same; but Birdy is stiffer:
Birdy vs Brompton Block by
1nterceptor, on Flickr
The Lake at Central Park, NYC by
1nterceptor, on Flickr
Birdy Suspension Block by
1nterceptor, on Flickr