Such a corporation may have a capital stock or may do business on the mutual plan. No such mutual corporations shall be incorporated after the first of July, 1969, to transact any kinds of insurance business on the assessment plan requiring as a condition for granting insurance the signing of a premium deposit note by the insured, which note is given for the purpose of establishing a limit of liability to assessment. Any such company currently licensed and operating on such plans shall be restricted to the lines of business in which it was engaged on July 1, 1969, provisions of N.H. Rev. Stat. § 401:7 notwithstanding.