If the value of a reversionary or remainder interest in property is included under chapter 11 in the value of the gross estate, the payment of the part of the tax under chapter 11 attributable to such interest may, at the election of the executor, be postponed until 6 months after the termination of the precedent interest or interests in the property, under such regulations as the Secretary may prescribe.

At the expiration of the period of postponement provided for in subsection (a), the Secretary may, for reasonable cause, extend the time for payment for a reasonable period or periods not in excess of 3 years from the expiration of the period of postponement provided in subsection (a).

For authority of the Secretary to require security in the case of an extension under this section, see section 6165.