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I don't know if they were standard option or not, but my 81 had one when I got it. The key looks to be very old, but the cap is shiny silver and looks new. The key is gold and says Taylor USA, if that means anything. Someone probably bought it and put it on after it was purchased tho. I'd take some pics, but I don't have my digital camera with me right now.
Ken,
It was not standard or a factory option. You might term it a "dealer option." The locking cap on my 2001 Impala I negotiated as part of the selling price. It came from the Parts Department. Here is the cap on my Corvette, courtesy of Canadian Tire.