For purposes of determining entitlement to hospital insurance benefits under subsection (b), any disabled widow aged 50 or older who is entitled to mother’s insurance benefits (and who would have been entitled to widow’s insurance benefits by reason of disability if she had filed for such widow’s benefits), and any disabled widower aged 50 or older who is entitled to father’s insurance benefits (and who would have been entitled to widower’s insurance benefits by reason of disability if he had filed for such widower’s benefits), shall, upon application for such hospital insurance benefits be deemed to have filed for such widow’s or widower’s insurance benefits.