That nipple connects to a nipple on the top corner of the radiator tank. I believe it's to remove trapped air. My original radiator has this fitting, but my replacement radiator does not. So the nipple on my coolant tube is capped too.
If this isn't clear, I can take a picture of the original tonight - yes, I still have the original radiator taking up space in the garage. My wife accuses me of being a pack rat for some reason.
Ciao!
Garth
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