Actually Les didn't start at Merc until sometime in the mid 70's after the 2 teams were consolidated and Merc was racing the T-3. I think the type IV and type VI SSM's are credited to him.
I have found the difference between the first Twister and the Twister-1 as well. The original Twister featured the "cow bell" side exhaust that turned down the side of the motor in a side chamber but did not reenter the leg; instead it goes straight out the bottom of the side chamber. The Twister-1 has the redesigned leg where the exhaust comes out to a side chamber and reenters the housing and exits through the SuperSpeedmaster's exhaust outlet below the cavitation plate.