My old Cyclo 3 cog cluster , Made in the 50's in England Used an 1/8" Chain ..
Brompton 2 & 6 speed use the 9 spline Driver of a Shimano BMX devised standard .. equal splines
this looks like its got the wide narrow splines of regular Hyper glide cassette driver s
https://secure.flickr.com/photos/elieser74/13990802776/
S-A S3X threads the spline tips .. so its a regular English freewheel /adjustable cup /bb thread. 1.37"