It is claimed that in the 1930's some of these labels which were normally used for testing
vending machines were accidentally dispensed to a customer. Several rolls of the labels
came into the market and were used on mail. The post office initiated an investigation,
prohibited the use of the labels on mail and announced that post due would be charged on
mail franked with the labels. To avoid confusion, the green label was replaced with a
grey label that was clearly inscribed "FOR TESTING PURPOSES ONLY" (Williams 125).