Section 4191, act Aug. 16, 1954, ch. 736, 68A Stat. 491, imposed a tax equivalent to 10 percent of the selling price upon over fifty specified office and business machines including adding machines, bookkeeping machines, cash registers, punch card and computing machines, typewriters, and tabulating machines. Section 4192, acts Aug. 16, 1954, ch. 736, 68A Stat. 491; Sept. 2, 1958, Pub. L. 85–859, title I, § 114(a), 72 Stat. 1278, granted an exemption for cash registers used in registering over-the-counter retail sales and for stencil cutting machines.