Jane,  it sounds like a whisky bottle.  4/5ths on the bottle used to be known
as a "fifth" of whisky (1/5th of a gallon), or 4/5ths of a quart.  The math
is there.  I used to bartend in Niagara Falls, NY, right across the border
during the late '60s.  That's how whiskey was sold before the bottlers and
distributors went first to the full quart and then to the liter on the
American side.
David Guild
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Univ. West Florida