Most master cylinders for use with power brake boosters used a rear piston with a shallow hole for the booster-to-master pushrod, as the rod was captive and couldn't fall out. There were other applications (like '67 Corvette, among others) that used the same master cylinder for both manual AND power brakes, with a rear piston with a deep hole, which was required for use with manual brakes, so the pedal pushrod couldn't fall out of the rear piston on the manual application, and the booster for use with that master cylinder used a long booster-to-master pushrod. Photo below shows both the short and long booster pushrods.