ref : Brompton Book , in print.

New is Mk 4, SRAM made the hubs for Mk3, UK sturmey AW3 or a few 2 cable 5 speed. is Mk2.
1st generation looked a lot different..
my Mk 2 has forged hinges, butt fillet brazed on, newer ones use a cast hinge, tubes fit inside
brass flows inside the space between. those got the extra inch of wheelbase.