Payment of premiums, and interest on premiums at the rate specified in section
406 [section
546 of this Appendix], which become due on a policy under the protection of this title [sections
541 to
549 of this Appendix] is guaranteed by the United States. If the amount guaranteed is not paid to the insurer before the period of insurance protection under this title expires, the amount due shall be treated by the insurer as a policy loan on the policy.