When I worked at Raybestos I noted that most if not all the disc brake mopar master cylinders from the 60s and 70s had the groove for the rubber pushrod retainer.
If the '67 master cylinder doesn't have it, they changed the master cylinder for disc C bodies in '69 to the same style used in B body hemis, that definitely would have the groove as there was a manual disc application of it used on taxis. And it's the same bore diameter 1-1/8".