Section 43O. The following corporations owning or operating cemeteries shall not sell or engage in the business of selling monuments for cemetery lots:? Municipal corporations, cemetery corporations organized under general laws, and cemetery corporations organized by acts of incorporation passed since March eleventh, eighteen hundred and thirty-one. This act shall be deemed an amendment of any charters heretofore granted except charters in which the right to sell monuments is expressly conferred. This act shall not prohibit the sale by any of said corporations of monuments in their possession prior to the effective date of this act.