For at least the 68's, black colored base and a square hole are generally associated with the jacks installed on Fremont cars. But you may be on to something regarding different years since all the bases you show have a square opening. Don't have a good answer for that unfortunately.
Scott please check your PMs. Been wanting to buy one of your jacks.
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Ed
1968 GTO (Thanks Mom)
2006 Silverado
2007 Cadillac SRX
2015 Chevy Express 
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