Proportion valve and Residual valve?

Found one for A body's on Ebag a drum brake distribution block $59 bucks new. Did not see one on Dr. Diff web page maybe something he carries but does not list or my computer unsavyness could not find.
Do not want to worry about 40 yr old crud in a used one and brakes are something I do NOT want to mess up.
Thanks for all the help!!!