Some lighters made between 1968 and 1972 have a "Solid Fuel Cell" rather than the usual cotton wool packing. This block is made from pressed rayon and was possibly trialled to speed up the process of inserting the packing. The problem is that the wick cannot penetrate the cell and has to be 'bunched' up at the top of the insert. This reduces the amount of contact area with the wick and therefore the amount of fuel that the wick can absorb.