It is immaterial, for purposes of this exception, whether the exemption from the employer tax on December 31, 1950, resulted, or would have resulted, from a tax exemption as such in effect on December 31, 1950, or from the provisions of section 1426(b) (6) of the Internal Revenue Code of 1939 in effect on that date, relating to the exception from employment of services performed in the employ of certain instrumentalities of the United States.
26 C.F.R. §31.3121(b)(6)-1