KELSEY HAYES PROPORTIONING VALVE KIT SOURCE

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.