I should know better to question Yoda, but what does 6-B-3-O-1 mean
but "except for Ch 8 sec 1 and 3 (prelim and adjusted address) the entire left arm, the clubshaft, and the back of the left hand are ALWAYS positioned against the same flat plane - the plane of the left wrist cock motion."
The "plane of the left wrist cock" is not the same concept as the inclined plane. It is the vertical hammer like cock and uncock of the left wrist with the Left Arm Flying Wedge maintained throughout. The swingers adjusted address where the left wrist is bent and the right wrist is flat being the exception to the maintenance of the LFFW. The hitter starting at Fix on the other hand sees the LFFW held in alignment while the left wrist cock goes from level to cocked to fully uncocked.
The left arm, assuming #3 angle is not zero, is not lying on the inclined plane. Golfers who place the grip more in the palm of the left hand for putting or short chips (basic motion), zero out their #3 angle for power regulation and will then have their left arm on the inclined plane.