In 1967 all cars went to dual masters, and disc brakes were an option. Older cars had a single master with drum brakes. On the right is a '66 distribution block that cannot be upgraded to the dual master because there is no inlet for the second line. The block on the left is also a distribution block but for the '67 dual master cylinder. It does have the provision for the second master line but cannot be used on disc brakes alone because it does not proportion any fluid; it's still just a distribution with a low-pressure warning light lead.