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

RSD-PV11_ml.jpg
the add on rear valve
92617958_L_1cb5af4d.jpg
then the disk block with the rear valve included.
RSD-PV12_ml.jpg
... if you have sliding front calipers they added a metering block to the front lines.
sum-760184_ml.jpg