N.J. Stat. § 43:10-57.1
Any probation officer of any county now or hereafter having a population of more than two hundred seventy-five thousand and not more than four hundred thousand inhabitants, who, at the time of his entry into service as a probation officer, had not attained the age of fifty years and who is not a member of the pension fund for probation officers of said county and who signifies in writing his desire to become a member of said pension fund to the board of chosen freeholders of the county within six months after the effective date of this act shall be admitted as a member of said pension fund, with the same standing as a member of said fund as if he had joined it upon his entry into such service as a probation officer; provided, (a) that he satisfies the board of chosen freeholders that he meets the conditions required under this act; (b) that he shall pay into said fund all accrued contributions, with interest, either in one payment or in regular semimonthly payments, under such rules and regulations as the board of chosen freeholders shall make; and (c) that he consent to the regular deductions required of other members of said fund.
N.J.S. § 43:10-57.1