A = Three phase B = Single phase T = Dual tone S = Series (A1, A2, B, C) Examples: A Thunderbolt 1000ASA2 would mean a three phase, series A2 Thunderbolt 1000. A Thunderbolt 1000BTSAC would mean a single phase, series C Thunderbolt 1000T. Most enthusiasts don't bother with the series designations, ...