This cog works with a standard Gates belt (centertrack or not), and mounts on standard hyperglide splines.
The cog should be printed in stainless steel. Because the dimensional accuracy of the stainless steel process is not quite good enough, it appeared to be impossible to get a good fit to a hyperglide spline. (Even printing it slightly undersized, and then filing it out very carefully to fit the hub splines resulted in annoying rattling within a day or two of riding.)
The plastic cog (here
) worked well -- good fit, no rattling, and sufficiently robust -- but was quite worn within a year-and-a-half of use. (See more at http://dkprojects.net/belt_drive_brompton
This stainless steel cog has a companion plastic "interface"
that fits between the cog and the hyperglide spline. Since the plastic is slightly deformable, it accommodates the dimensional inaccuracy of the stainless steel part.
It's a little difficult to assemble on the hub, but it works beautifully! No skipping. No rattling. My experience with other stainless steel cogs indicates it should last many years.
The cog includes printed-threaded holes for three M3 (diameter) x 0.5 mm (pitch) x 4 mm (length) set screws, which engage the groove on the hub and keep the cog from sliding axially along the splines. Again, the threads are not fully accurate (and their size is rather small for the print resolution). I ran a tap through the holes; it took off surprisingly-little metal, but did shine up/sharpen up the threads, which then worked fine.
To mount on the hub, first put the plastic interface onto the hub -- but not quite all the way. Leave a couple of millimeters overhanging the edge of the splines. This gives a little bit of the interface that can be squeezed inside the cog splines (make sure the set-screw holes line up!). Then you can use the axle bolt with some washers/spacers to slowly press the cog and interface onto the hub splines. Insert and tighten the set screws, and you're done! (Again, some details are at http://dkprojects.net/belt_drive_brompton).