KELSEY HAYES PROPORTIONING VALVE KIT SOURCE

Mattax said
Its not like its easily adjustable like the aftermarket ones with a knob.

Here's a discussion about which cars it came on.
Kelsey Hayes Brake Bias Adjuster - CorvetteForum - Chevrolet Corvette Forum Discussion

Note that does not indicate these cars came with the K-H 4 piston calipers. Just that they came with this prop valve made by K-H.

Appears that article has things to do with a "RACE CAR" and nothing to do about with an assembly line, street driven, automobile.
Some RACER doing his best in MODIFYING a valve to suit his racing needs.