It just protects the terminal and nut. That's it. Keeps errant wrenches from accidently contacting it. Although it's only snapped on to the little "ring" at the base. It's easier to get knocked off of there than you might think. Perhaps that's how yours got missing?
The 497628 was a black cap from GM. They used it in the 70s and 80s.
The 25523333 cap was a red cap. When they got rid of that in 1990, the black cap replaced the red cap. That's all the info I can ascertain at the moment.
So I'm THINKING that G-bodies came from the factory with red ones, but the replacements in 1990 reverted back to the "old" part number, a black cap. The color is the only difference I can find at the moment. There may be other subtle differences between the two.
The black 497628 repro from Gbodyparts is the best you can do for new, at least for now.
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