When these Revenue Stamps were first issued, it was required that each type of stamp was used on the correct type of item – playing card stamps on playing card packs, contract stamps on contracts, Proprietary Stamps on proprietary items, etc. However, by the end of that first year, it was increasingly difficult to keep stocks of all the various stamps on hand, so the rules were loosened and some stamps could be used on items they were not issued for. The notable exception to the new rule was Proprietary Stamps.