Amounts paid or accrued on indebtedness incurred or continued, directly or indirectly, to purchase or to continue in effect a single premium life insurance or endowment contract, or to purchase or to continue in effect a single premium annuity contract purchased (whether from the insurer, annuitant, or any other person) after March 1, 1954, are not deductible under section 163 or any other provision of chapter 1 of the Code. This prohibition applies even though the insurance is not on the life of the taxpayer and regardless of whether or not the taxpayer is the annuitant or payee of such annuity contract. A contract is considered a single premium life insurance, endowment, or annuity contract, for the purposes of this section, if substantially all the premiums on the contract are paid within four years from the date on which the contract was purchased, or if an amount is deposited after March 1, 1954, with the insurer for payment of a substantial number of future premiums on the contract.