The buttons in this picture are cream color.
So short answer ..
Buy a Chinese 30 series driver with the bore size for what ever motor your putting it on.
Always buy genuine Comet belts ... pick yours from the pictured chart.
Measure your driven outside diameter.
Measure the center distance between the driver and the driven and choose your belt.
*NOTE*
Your driver and driven need to line up.
the inboard side of both dosent move ...
Use a straight edge to make sure they do line up .. if they don't, shim them till they do.
Easy Peasy Lemon Squeezy!