porpotion valve
Sorry I couldn't find a part number for the 69 valve.
Do you have a picture of your current distribution block? If the disc brakes were added a proportioning valve would be added into the rear exit of the block OR the block changed to one with the internal valve.
here is a pic of a manual block
the add on rear valve
then the disk block with the rear valve included.
... if you have sliding front calipers they added a metering block to the front lines.