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https://www.condorcycles.com/products/sturmey-archer-1-8-3-spline-flat-sprocket shows a 13t 1/8” sprocket in stock.
https://www.sjscycles.co.uk/sprockets/sturmey-archer-13t-sprocket-18-flat-hsl854/ similarly.
Yes but for the 6-speed SRAM/SACHS set up, the width of the outer sprocket by the hub needs to be 3mm with the teeth narrowed down to take the 3/32" chain. Here is the parts diagram with dimentions for QRSPRSTACK-SR6:
https://issuu.com/bromptonbicycle/docs/qrsprstack-sr6_as
https://www.condorcycles.com/products/sturmey-archer-1-8-3-spline-flat-sprocket shows a 13t 1/8” sprocket in stock.
https://www.sjscycles.co.uk/sprockets/sturmey-archer-13t-sprocket-18-flat-hsl854/ similarly.