A prisoner serving a sentence of life imprisonment, except one convicted of murder in the first degree, one convicted of murder which was psycho-sexual in nature and committed prior to April 15, 1974, or one sentenced under N.H. Rev. Stat. § 632-A:10-a, III, may be given a life permit at any time after having served 18 years. Eighteen years shall be deemed the minimum term of his sentence for the purposes of this section, minus any credits received pursuant to N.H. Rev. Stat. § 651-A:23, plus the disciplinary period added to such minimum under N.H. Rev. Stat. § 651:2, II-e, any part of which is not reduced for good conduct as provided in N.H. Rev. Stat. § 651-A:22, provided that there shall appear to said board to be a reasonable probability that he will remain at liberty without violating the law and will conduct himself as a good citizen. The provisions of this section shall not apply to a prisoner serving a life sentence when the court, pursuant to N.H. Rev. Stat. § 630:1-b, II, has specified a minimum term other than that prescribed in this section.