Therear bag is made\ for Brompton by RAdical Design. I have a slightly "grey" import from the very early days.
IT is big and roomy, fitting to the rack with a loop that fits over the bung and a pair of straps which fit at the rear of the rack a buckle clipping underneath. Mine is elastic, but I believe the new ones have straps which alow tightening.
When riding, the bag is stable, providing anything heavy is at the bottom , if a one sided top heavy packing is used it has a tendency to tip to one side. Even then however it is still clear of the heels.
My only problem is that a non standard light interfered withthe rear strap, but tucking it behind the light worked.
In all it does not replace the excellent front bag on the T6, but on the S6 with the smaller front bag it helps with shopping and weekends away.
There was a distribution problem last year as Brompton embargoud Humbert and then weren't marketing them!