filler plug
originals were not vented
originals had a mopar oily grease concoction in them. might have been multi mileage lube might not.
closet you can get today is Texaco/Chevron MARFAC but they seem to sell that stuff in 40 gallon drums.... you'd be investing $$$ hundreds for a job that needs a cup full
maybe when filled with modern grease or a grease/90 weight gear oil cocktail they need venting....
id thread it and put a grease nipple/zerk fitting in it otherwise when your headers heat it up its going to ooze out