Notwithstanding any statute or rule of law to the contrary, town and city trustees of trust funds may establish, maintain and operate one or more common trust funds, in which may be combined money and property belonging to the various trusts in their care, for the purpose of facilitating investments, providing diversification and obtaining reasonable income; provided however, that said common trust funds shall be limited to the investments authorized in N.H. Rev. Stat. § 31:25; provided further, that not more than $10,000, or more than 10 percent of the fund whichever is greater, of any town or city common trust funds shall be invested under N.H. Rev. Stat. § 31:25 in the obligations of any one corporation or organization, excepting deposits in any federally insured bank authorized to accept deposits under N.H. Rev. Stat. § 6:8, I and I-a, in credit unions in this state, or in obligations of the United States and of the state of New Hampshire and its subdivisions; or in participation units in the public deposit investment pool established pursuant to N.H. Rev. Stat. § 6:45, or in shares of open ended mutual funds selected by the trustees for investment under N.H. Rev. Stat. § 31:25, and provided further, that the participating contributory interests of said trusts are properly evidenced by appropriate bookkeeping entries showing on an annual basis the capital contribution of and the profits and income allocable to each trust.