An estate or trust is allowed the credit against tax for partially tax-exempt interest provided by section 35 only to the extent that the credit does not relate to interest properly allocable to a beneficiary under section 652 or 662 and the regulations thereunder. A beneficiary of an estate or trust is allowed the credit against tax for partially tax-exempt interest provided by section 35 only to the extent that the credit relates to interest properly allocable to him under section 652 or 662 and the regulations thereunder. If an estate or trust holds partially tax-exempt bonds and elects under section 171 to treat the premium on the bonds as amortizable, the credit allowable under section 35, with respect to the bond interest (whether allowable to the estate or trust or to the beneficiary), is reduced under section 171(a)(3) by reducing the shares of the interest allocable, respectively, to the estate or trust and its beneficiary by the portion of the amortization deduction attributable to the shares.