United States Government life insurance shall be issued against death or total permanent disability in any multiple of $500 and not less than $1,000 or more than $10,000. No person may carry a combined amount of National Service Life Insurance and United States Government life insurance in excess of $10,000 at any one time. The limitations of this section shall not apply to the additional paid up insurance the purchase of which is authorized under .section 1907 of this title