(i) General rule. All small FSCs which are members of a controlled group on a December 31, shall, for their taxable years which include that December 31, be limited to one $5 million amount. The $5 million amount shall be allocated equally among the member small FSCs of the controlled group for their taxable years including that December 31, unless all of the member small FSCs consent to an apportionment plan providing for an unequal allocation of the $5 million amount. The apportionment plan shall provide for the apportionment of a fixed dollar amount to one or more of the corporations, and the sum of the amounts so apportioned shall not exceed the $5 million amount. If the taxable year including the December 31 of any member small FSC is a short period (as defined in section 443), the portion of the $5 million amount allocated to that member small FSC for that short period under the preceding sentence shall be reduced to the amount which bears the same ratio to the amount so allocated as the number of days in such short period bears to 365. The consent of each member small FSC to the apportionment plan for the taxable year shall be signified by completing the form (i.e., Schedule O or any successor to that form) which satisfies the requirements of and is filed in the manner specified in 1.1561-3. An apportionment plan may be amended in the manner prescribed in 1.1561-3(a), except that an original or an amended plan may not be adopted with respect to a particular December 31 if at the time the original or amended plan is sought to be adopted, less than 12 full months remain in the statutory period (including extensions) for the assessment of a deficiency against any shareholder of a member small FSC the tax liability of which would change by the adoption of the original or amended plan. If less than 12 full months of the period remain with respect to any such shareholder, the director of the service center with which the shareholder files its income tax return will, upon request, enter into an agreement extending the statutory period for the limited purpose of assessing any deficiency against that shareholder attributable to the adoption of the original or amended apportionment plan.