I had a good look today and found the pedal return stop is indeed the brake light switch.
However, there is free play there. In other words, I can move the pedal forward (depressing it) a bit before it moves the booster rod. So, the actual MC push rod stop is in the booster ass'y.

Out of desperation I drilled the MC piston, where the booster rod engages, 2mm deeper. That fixed the problem. (for now).
To test for "punch thru" I made several "stomps" on the brake pedal. So far, so good.

I'm wondering if there was a different MC for this particular chassis.

Stumped this chump!