Surly BMX Single Cassette Cog
Made with heavy-duty SCM 415 cromoly steel. This is hardened cromoly; looks good, lasts a long time. Designed to fit Shimano freehub bodies. The Shimano freehub spline is the most common design out there, used on Shimano brand and many non-Shimano hubs. You're likely using this cog to convert your current wheel to single speed. The footprint of the cogs (where they fit onto the cassette body) is wider than most single speed cogs so they don't dig into the cassette body and get sloppy.