KELSEY HAYES PROPORTIONING VALVE KIT SOURCE

I believe that is a Proportioning Valve for an early J-56 brake system that came out on the C-2 corvette. My 66 has that option.
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that appears to be a Corvette J-56 proportioning valve....66-67 Corvettes with J-56, came with that valve, and maybe 68's, but 69 J-56 cars did not use it.


Those are quotes from the linked thread.
Clearly at least some Corvette owners believe that proportioning valve came from the factory - and yes for street use.