Yet another balk question

[TwoBits]

Wording of FED rule 6-2-4f regarding infractions that result in a balk:

"...failing to pitch to the batter when the entire non-pivot foot passes behind the perpendicular plane of the back edge of the pitcher's plate, except when feinting or throwing to second base in attempt to put out a runner."

If the pitcher's foot landed "on the backside of the rubber" as described in the OP, then it did pass behind the "perpendicular plane of the back edge of the pitcher's plate". Remember your geometry: planes extend indefinitely, and the "perpendicular plane of the back edge of the pitcher's plate" would not be limited to just the 24" width of the pitcher's plate.

Having said all that, I doubt I would have been able to see enough to call a balk from either behind the plate or from the B position.