(b)
Any member (other than one who has elected not to be insured under this subchapter for the period or periods of duty involved)—
(1)
who, when authorized or required by competent authority, assumes an obligation to perform (for less than thirty-one days) active duty, or active duty for training, or inactive duty training scheduled in advance by competent authority; and
(2)
who is rendered uninsurable at standard premium rates according to the good health standards approved by the Secretary, or dies within one hundred and twenty days thereafter, from a disability, or aggravation of a preexisting disability, incurred by such member while proceeding directly to or returning directly from such active duty, active duty for training, or inactive duty training as the case may be;
shall be deemed to have been on active duty, active duty for training, or inactive duty training, as the case may be, and to have been insured under this subchapter at the time such disability was incurred or aggravated, and if death occurs within one hundred and twenty days thereafter as a result of such disability to have been insured at the time of death. In determining whether or not such individual was so authorized or required to perform such duty, and whether or not such member was rendered uninsurable or died within one hundred and twenty days thereafter from a disability so incurred or aggravated, there shall be taken into account the call or order to duty, the orders and authorizations of competent authority, the hour on which the member began to so proceed or to return, the hour on which such member was scheduled to arrive for, or on which such member ceased to perform such duty; the method of travel employed; such member’s itinerary; the manner in which the travel was performed; and the immediate cause of disability or death. Whenever any claim is filed alleging that the claimant is entitled to benefits by reason of this subsection, the burden of proof shall be on the claimant.