Its been my personal experience (68 GTO) that in the late 60s, the black jack bases were from the Fremont plant. Fremont had a different design jack head, base (square opening) and colors.
Not sure about the other plants but its another area that the GTO Restoration Guide was wrong in assuming all were Light Blue.
Photos of my original jack.
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Ed
1968 GTO (Thanks Mom)
2006 Silverado
2007 Cadillac SRX
2015 Chevy Express 
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