Credit unions (other than Federal credit unions described in section 501(c)(1)) without capital stock, organized and operated for mutual purposes and without profit, are exempt from tax under section 501(a). Corporations or associations without capital stock organized before September 1, 1951 and operated for mutual purposes and without profit for the purpose of providing reserve funds for, and insurance of, shares or deposits in:
are also exempt from tax under section 501(a). In addition, corporations or associations of the type described in the preceding sentence which were organized on or after September 1, 1951, but before September 1, 1957, are exempt from tax under section 501(a) for taxable years beginning after December 31, 1959.
26 C.F.R. §1.501(c)(14)-1