When was that? Not the formed cap, but flat and glued on? Sounds like the plastic bottles in convenience stores maybe in Jersey or NYC? Trying to remember, I think that's how the ones I used were, but I've seen the earlier plain cardboard circles with a little tab that just pressed into the opening. Never had it at home but in the neighborhoods it was still common, and for years after delivery stopped you'd see the boxes on front steps, like they hoped the milkman would return?