Such insurers admitted to do business in this State under authority of this article shall be incorporated as stock or mutual companies as defined in section 3302 of this title. Except as provided in sections 4831 through 4856 of this title, no unincorporated or joint stock association shall be admitted after July 1, 1968, for the purpose of doing any insurance business in Vermont. Such unincorporated or joint stock associations which are lawfully admitted and qualified on such date may continue to carry on their business in this State subject to the provisions of this part, but they shall not write any new or additional lines of insurance after such date. (Added 1967, No. 344 (Adj. Sess.), § 1 (ch. 1, subch. 2, § 5); amended 1971, No. 31, § 3, eff. March 31, 1971.)