Doesn't matter on the Holley Dual feed. I am dusting cobwebs of my brain about the H and HH. Orifice comes to mind for flow rates. Anybody else wants to jump in here, it would be appreciated. The standard needle and seat assemblies for these things have the same height and O-ring placement, so as for astectics are concerned, it doesn't matter because they will work in each hole. Just lube them with crc or wd-40 or what ever cranks your scooter before placing them in the hole.