Get one for a 1968 dart GTS 340 manual disc from napa. It will have a tab with a screw that holds the piston with dual diaphragm assembly in place.
Here https://www.napaonline.com/en/p/NMBP1877M
I just upgraded to the lighter aluminum mp/strange master cylinder. I had the napa one linked above and it worked great, just wanted less weight.
