Section 3112 makes ineffectual as to the employer tax imposed by section 3111 those provisions of law which grant to an instrumentality of the United States an exemption from taxation, unless such provisions grant a specific exemption from the tax imposed by section 3111 by an express reference to such section or the corresponding section of prior law (section 1410 of the Internal Revenue Code of 1939). Thus, the general exemptions from Federal taxation granted by various statutes to certain instrumentalities of the United States without specific reference to the tax imposed by section 3111 or by section 1410 of the 1939 Code are rendered inoperative insofar as such exemptions relate to the tax imposed by section 3111. For provisions relating to the exception from employment of services performed in the employ of an instrumentality of the United States specifically exempted from the employer tax, see §31.3121(b)(5)-1. For provisions relating to services performed for an instrumentality exempt on December 31, 1950, from the employer tax, see paragraph (c) of §31.3121 (b) (6)-1.