Update: I rode by now with the pack lightly filled, heavily filled and medium. I rode uphill and downhill, sidewalks, street, gravel, etc. There was neither shimmy, nor other problems. Two suspension points appear sufficient. I was expecting problems riding side-by-side with others and passing pedestrians. No problems whatsoever.

The pack is more practical than the T-bag or Borough. It leaves more open space by the handlebars and beats the Brompton-branded bags off the bike by miles.

Maybe, I should add a potential correction. The vendors use the term S-bag frame for all the frames they sell, but I doubt they can all relate to the particular bag. The designation they use to distinguish the particular frame from other is RX30x20, which stands for 30cm height, 20cm width and no handle.

At this point I turn to potential refinements. I look at some quicker attaching of the frame to the pack and making that frame slightly curved to make it tuck in better within the backpack.