[12 piece bucket]

The G.O.L.F. Strong grip assumes both the #3 Pressure Point and the Left Thumb to be Aft. With Strong Single Action with the Left Wrist Vertical the requirement is for there to be "separation" between the Left Thumb and Left Forefinger (fanger for y'all from down South). This separation is required because the Left Wrist is Vertical.

I think that a lot of pros do get their Left Thumb Aft. However this is achieved more due to the Left Wrist being Turned and thus no "separation."

Why the thumb aft? Per 10-2-0 . . .

Unless otherwise indicated the left thumb is always placed on the same line as Pressure Point #3, and covered with the cup of the right hand palm. With strong Grips, this places the Thrust of both the #1 and the #3 Pressure Points On Plane so that the Clubhead Lag alignments are established and can be verified by the Waggle and the Foward Press.