The purchase of a security from the portfolio of an open-end investment company holding itself out as a money market fund by any affiliated person or promoter of or principal underwriter for the money market fund or any affiliated person of such person shall be exempt from section 17(a) of the Act (15 U.S.C. 80a-17(a) ) ; provided that: (a) In the case of a portfolio security that has ceased to be an Eligible Security (as defined in § 270.2a-7(a)(12) ), or has defaulted (other than an immaterial