I've thought that I had it before, so I'm going to make it prove itself for longer before the victory dance.
I was asked back-channel "why not just use a m/c that doesn't have that thing on the side of it?" and the reason is that this is the OEM application m/c,, not something odd from some other vehicle that I won't remember in 3-5 years. FWIW one of my co-workers has a '67 F-250 cab on an '89 F-350 chassis. He transplanted all of the operating parts from the '89 onto the '67 cab. The whole m/c-booster assembly is the '89 application. I noticed when he was showing off the partially completed MicroSquirt based EFI conversion on the 300-6 (TFI dist. run bby the MS) that the '89's m/c was cast iron, but had this same RPV assembly screwed into the side of it. I can not recall having ever noticed this before.
The Valiant was a "Bitsa". Parts were a bit of this and a bit of that. It gets old when you need a part that you've already had to figure out what works, only now you can't recall what it's OE application was. I ended up starting a binder on the car to document what each part was.