Mass. Gen. Laws ch. 175, § 82

Mutual fire companies organized prior to May twenty-first, eighteen hundred and eighty-seven, and now lawfully doing business upon the plan of taking deposit notes for a percentage of the amount insured by their policies, and making a call or assessment thereon for expenses and for the payment of losses only after such losses are incurred, may continue such system of business, and such deposit notes shall constitute the entire liability of their members.