An insurer may invest in common stocks, other than insurance stocks, of any solvent institution organized and existing under the laws of the United States or Canada, or of any state or province thereof, if during a period of 7 fiscal years next preceding the date of acquisition by such insurer the institution had aggregate net earnings available for the payment of dividends upon its common stock of no less than the aggregate sum which would have been sufficient to pay dividends of 4% per annum upon the par value (or in the case of common stocks without par value, upon the stated capital) of all of its shares of common stock outstanding during such period. As used in this section the term “common stock” includes transferable certificates of participation in business trusts and statutory trusts. An insurer may invest in or otherwise acquire and hold a limited partnership interest in any limited partnership formed pursuant to the laws of any state or the United States of America. No limited partnership interest shall be acquired under this section if the cost thereof would exceed 2% of the assets of such insurer nor if such cost, plus the book value on the date of such acquisition of all limited partnership interest then held by such insurer and acquired under this section, would exceed 10% of such assets.