I think you just use the bottom 2 studs to mount the new master. Or you could get a new modern booster off of a newer car, check the bone yards to see what would fit.
I could be crazy! if I'm mistaken I apologize, are you sure that's not the plunger from inside the master cylinder, with the push rod clipped inside that plunger.
Joe, that is a Bendix 1-5/8 push rod hole. A 67 to 70 Caddy except 68 to 7o Eldorado also uses the same size push rod but don't know if it's a 4 bolt mount. Maybe take yours to an auto store and try to match it up. Hope this helps.
OMG! You might be crazy, but you've got a good eye! That is way too fancy looking to just be a master cylinder push rod. And what's up with the little, tiny holes drilled around the face of the end of it?
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