United States Postage Meter Stamp Catalog/Surcharge slugs
Beginning in 1963 new postal rates required unusual postage values that incorporated fractions of a cent. None of the meters in use at the time could print these fractional values. While new more versatile meters were being developed meter manufacturers supplied customers with surcharge slugs to show that the extra fractional value was accounted for. Meter users paid for the fractional values by providing the Post Office with sheets of paper with meter stamps affixed totaling that fractional values used.
Surcharge slugs usually included the mail classification such as "Bulk Rate" or "Nonprofit Organization" as well as the fractional value.