Additional credit under section 3302(b) shall not be allowed against the tax for any calendar year unless there is submitted—

(a) To the Commissioner a certificate of the proper officer of each State (with respect to the law of which the additional credit is claimed) showing the highest rate of contributions applied under the State law in such calendar year to any person having individuals in his employ; and

(b) To the district director a certificate of the proper officer of each State (with respect to the law of which the additional credit is claimed) showing for the taxpayer—

(1) The total remuneration with respect to which contributions were required to be paid by the taxpayer under the State law with respect to such calendar year; and

(2) The rate of contributions applied to the taxpayer under the State law with respect to such calendar year.

If under the law of such State different rates of contributions were applied to the taxpayer during particular periods of such calendar year, the certificate shall set forth the information called for in paragraphs (b)(1) and (2) of this section with respect to each such period.

(c) Such other or additional proof as the Commissioner or the district director may deem necessary to establish the right to the additional credit provided for under section 3302(b).