For purposes of determining entitlement to hospital insurance benefits under subsection (b) in the case of an individual under age 65 who is entitled to benefits under section 402 of this title , and who was entitled to widow’s insurance benefits or widower’s insurance benefits based on disability for the month before the first month in which such individual was so entitled to old-age insurance benefits (but ceased to be entitled to such widow’s or widower’s insurance benefits upon becoming entitled to such old-age insurance benefits), such individual shall be deemed to have continued to be entitled to such widow’s insurance benefits or widower’s insurance benefits for and after such first month.