The Secretary shall, except as hereinafter provided, upon application by the insured and proof of good health satisfactory to the Secretary and payment of such extra premium as the Secretary shall prescribe, include in any National Service Life Insurance policy on the life of the insured (except a policy issued under section 620 of the National Service Life Insurance Act of 1940, or section 1922 of this title) provisions whereby an insured who is shown to have become totally disabled for a period of six consecutive months or more commencing after the date of such application and before attaining the age of sixty-five and while the payment of any premium is not in default, shall be paid monthly disability benefits from the first day of the seventh consecutive month of and during the continuance of such total disability of $10 for each $1,000 of such insurance in effect when such benefits become payable. The total disability provision authorized under this section shall not be issued unless application therefor is made either prior to the insured’s fifty-fifth birthday, or before the insured’s sixtieth birthday and prior to January 1, 1966. The total disability provision authorized under this section shall not be added to a policy containing the total disability coverage heretofore issued under section 602(v) of the National Service Life Insurance Act of 1940, or the provisions of this section as in effect before January 1, 1965, except upon surrender of such total disability coverage, proof of good health, if required, satisfactory to the Secretary, and payment of such extra premium as the Secretary shall determine is required in such cases. Participating policies containing additional provisions for the payment of disability benefits may be separately classified for the purpose of dividend distribution from otherwise similar policies not containing such benefits.