“ The parties stipulated that the statute applied only to 'self-contained air conditioning units of the household type' and that each of the two units in question had an actual motor horsepower of one horsepower. The taxpayers contended that the words 'motor horsepower' in the revenue rulings meant actual horsepower; the Government contended that they meant the nominal horsepower given by the manufacturer or 'rated' horsepower assigned on the basis of standards established by trade associations. ”

“ And we add that our disposition is without prejudice to such action as the lower courts may deem appropriate to prevent taxpayers, should they ultimately prevail, from obtaining a windfall by reason of taxes collected by them but not paid to the Government. ”

“ This suit was instituted by petitioners in the District Court for a refund of excise taxes collected on the sales of two air-conditioning units sold in 1954 and 1955. Section 3405 (c) of the Internal Revenue Code of 1939, 26 U.S.C. (1952 ed.) § 3405 (c) , 26 U.S.C.A. § 3405 (c) , placed a 10% tax on ' (s) elf-contained air-conditioning units.' [1] Section 3450 gave the Commissioner, with the approval of the Secretary, power to prescribe needful rules and regulations for the enforcement of the provisions relating to such taxes. ”