El Salvador 1930 Bolivar Centenary Airmail Set M Mint. Scott C15-C18 with variety

The 15c value of this stamp has a 15c double variety that is catalogued at a 1300% premium. Brian Moorhouse argues in the Mainsheet that these stamps may well have been duty and key plate printed. The 15c stamp in this particular set would seem to evidence that, as the numeral can be seen to be printed in a much darker shade of red (as quoted in Mainsheet) when compared with the regular 15c stamp. This is the only copy I’ve seen with this colouration. Additionally, this set includes the 40c value in a darker shade of blue than is found on the regular stamp.These stamps were hand gummed and hand perforated (hence typical irregular perfs). Very undercatalogued with only 5,000 sets printed.