current 5 speed is single cable , at it's core its the AW/BSR 3/4 , 4/3 gear, and another set of planetaria
gears for 1st & 5th..

Sach's 3 speed was the source of re-designing the extended driver to fit a 13 & 15 t 3 spline cog ..

Iv'e been using the 15t which has 3 short shift gate teeth and is 3/32" thick, on my BSR..
3/32 chain


FWIW
the old 5speed used 2 pull chains the 234 ratios a little tighter spaced ,
but those are said to have some quirks ... gear slipping, mis shifts..

now the SA 5 speed is using the 9 spline driver same as the BWR, and cassette freehubs , sorta,
and 2 cog fittings are posible SunRace SA
given12,13-15, 16 t .. as Brompton spares 13-15 from the BWR
the 12 - 16 pair coming from the 2 speed non IGH hub kit.

some bigger ones from te S3X hubs, also 9 spline..

Some machine shop access needed, but says here... you can fit 3 cogs on and run 10 speed chain spacing.
http://forum.ctc.org.uk/viewtopic.php?f=5&t=55908