This "pulley" allows use of a Gates "CarbonDrive" belt with a Sturmey-Archer hub. See dkprojects.net/belt_drive_brompton
This gives me a higher gear ratio than the 20-tooth pulley available from carbondrive.com. Printed in stainless steel, it works great.
One caution: Gates says 20 teeth is their minimum recommended pulley size. "With smaller pulleys the fibers may be damaged under heavy loads and the belt could fail." I'm betting that the belts have to be designed for a stronger, heavier rider than I (135 lbs). So far so good.
Also, because the dimensional accuracy of stainless steel printing is not great, the center diameter is a bit undersized -- I had to file by hand for about an hour to get it to fit!