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There is a double space (5 M) on line 3 During 1951, because of the conflict in South Korea, brass again became a difficult commodity to obtain.Zippo switched to using chrome plated steel for their lighters.As with the bottom stamps, close attention should be paid to the positioning of the word Zippo compared to the top and bottom lines.
The top picture is a chrome plated Zippo, whilst the bottom picture is a Nickel-silver unplated lighter.
This may have been caused by a greater pressure on the stamping machine, pushing further into the metal, so these could all be the same design, stamped with the same tool, but on different days.
It should be noted that styles H & I have only been found on steel lighters so the pressure of the stamping machine could have been increased to cope with the different material During the period above, there were a number of alterations made to the insert.
Looking at the "CO" it will be noticed that the height of the lines has shrunk as both these letters are more rounded.
The only difference is the thickness of the letters.