Years ago I ran into some power master cylinders that didn't have the groove in the piston for the manual push rod. Do they all have them now? It doesn't take much to knock out the studs.
I'm guessing he has the studs preinstalled- that's why the other listing is for "all A bodies", and "B bodies with power brakes"- because those applications have the studs in the pedal bracket/firewall and in the booster, nuts on the master.