It looks like the B&B 36XL037-6A4. I'm holding one right now.
The picture on the catalog page is not the sprocket
here
The B&B 36XL037-6A4 is the part in the KOP and the one you are asking about.
I'm thinking it mates to the Gates GT2 belts. The data sheet says XL which doesn't make sense. GT2 is a 5mm pitch and XL is a 0.2' pitch. It is very close and it has a different tooth profile.
Anyway that is the sprocket and it should mate to GT2.