1965 Valiant brake woes

Could be. But why on Earth would someone add a separate proportioning valve? I guess they wanted a lot of adjustment.
judging by the junky mismatched hardware and the "adjustable" push rod featured in the other post, i'd wager a guess that there was a fair bit of tom foolery going on.

your earlier guess might've been spot on. maybe they were gettin' ready to do discs? or they just wanted power and that was the easiest way to get there-- open up the catalog and fire up the credit card, press that easy button.