So the 69 A12 that I purchased was originally drum/drum. The previous owner installed a common front disc brake kit but continued to use the drum/drum master. I ordered a master from online tube that was listed as manual or power brake application. My concern is the piston does not have the machined groove to lock in the push rod. I called pirate Jack and asked about their master cylinder and he said it would also not have that groove. Is there a factory appearing master cylinder for manual disc brakes that has a groove to lock in the rod?