“ The fact that Congress enacts a statute containing a 'grandfather clause,' which exempts from the general income tax certain corporations organized prior to a specified date, does not of itself indicate that Congress has made an arbitrary classification. ”

“ We do not agree.
Prior to 1951, all savings and loan associations were exempt from taxation of income derived from their operations. Also exempt were nonprofit corporations that insured the savings institutions. In 1951, the exemption for savings and loan associations was discontinued, on findings that the industry had developed to a point comparable to that of commercial banks. The exemption for insurers, however, was continued, provided they were already in existence as of September 1, 1951. ”

“ Public Utilities Comm'n, supra, similarly Congress does not exceed its power to tax nor does it violate the Fifth Amendment when it refuses to exempt from tax newly formed corporations, the multiplication of which might burden otherwise valid federal programs. ”