Postal currency
Black's Law Dictionary · Henry Campbell Black, M.A. · 1910
Black's Law Dictionary
During a brief period following soon after the commencement of the civil war in the United States, when specie change was scarce, postage stamps were popularly used as a substitute; and the first issues of paper representatives of paris of a dollar, issued by authority of congress, were called "postal currency." This issue was soon merged in others of a more permanent character, for which the later and more appropriate name is "fractional currency."
Abbott.