The indenture trustee shall give to the indenture security holders, in the manner and to the extent provided in subsection (c) of section
77mmm of this title, notice of all defaults known to the trustee, within ninety days after the occurrence thereof: Provided, That such indenture shall automatically be deemed (unless it is expressly provided therein that such provision is excluded) to provide that, except in the case of default in the payment of the principal of or interest on any indenture security, or in the payment of any sinking or purchase fund installment, the trustee shall be protected in withholding such notice if and so long as the board of directors, the executive committee, or a trust committee of directors and/or responsible officers, of the trustee in good faith determine that the withholding of such notice is in the interests of the indenture security holders.