Master cylinder help needed

This is a picture of what I think is a Mopar MC, part no. 3461187. It's for a '71 Dart w/manual disc brakes. I have a rebuild kit for it and the FSM instructions, step #2 states, "Remove front piston retainer screw from inside reservoir . ." I see where the screw should be (red circle in pic) but it's not in this MC. This MC was a rebuilt unit by EIS. Does anyone know if I need to find this missing screw or is there some way to tell if the piston is retained by some other method?

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