Mont. Code Ann. § 19-1-824

On certification, the governor shall direct the department director to enter into a modification to the existing agreement with the appropriate officers of the federal government, pursuant to section 218 of the Social Security Act, 42 U.S.C. 418, to secure coverage for the retirement system with respect to which certification has been made. An agreement may be made retroactive to the extent permissible under the Social Security Act.