in the case of an individual who is entitled to monthly benefits under section 402 or 423 of this title on the first day of his initial enrollment period or becomes entitled to monthly benefits under section 402 of this title during the first 3 months of such period, his enrollment shall be deemed to have occurred in the third month of his initial enrollment period, and

in the case of an individual who is not entitled to benefits under section 402 of this title on the first day of his initial enrollment period and does not become so entitled during the first 3 months of such period, his enrollment shall be deemed to have occurred in the month in which he files the application establishing his entitlement to hospital insurance benefits provided such filing occurs during the last 4 months of his initial enrollment period; and