65 Barracuda Power Brake Master

That screw and tear drop washer come off so you can then bench bleed the master cylinder.

Then on mine (manual) there is a 2-bolt retainer that holds the rubber dust boot in place on that, which uses that hole with the screw in it and the opposite hole. My new master came with the boot.
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Others can chime in if power brakes are the same, but I suspect it is.