As you know, riding a fixed gear bike involves putting strong backward force on the pedals regularly to reduce speed. These “brake” forces are just as strong as when starting a sprint... Therefore track hubs have a counter thread for mounting a “lock ring” that prevents the sprocket to come loose.
When converting a nice road bike to fixed you can of course glue a normal track cog to your hub and secure it with a ring but then it will be very difficult to change the sprocket because you want an other gear ratio of because the cog has been worn out.
This solution uses a Miche track sprocket carrier that you permanently fix to the hub with metal–epoxy (2 component) glue.