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In a fuel tank, an upstanding tubular guide member extending through said tank but not communicating therewith, a movable member slidable longitudinally within said guide member, a normally inoperative pawl and ratchet device associated with said slidable member, and said guide member, magnetic flotation means outside of and guided vertically by said guide member adapted to move said pawl and ratchet device into positive relative operative engagement when the pawl piece comes within the influence of the field of said magnetic means and thereby limit the upward range of movement of said slidable member and means for indicating the movement of said movable member.