In states which have not elected in to Social Security, employees of state and local governments must nevertheless be covered by Social Security unless the employees are covered by a retirement plan that meets certain requirements ("FICA alternative plan").
Treas. Reg. § 31.3121(b)(7)-2 gives the details. So long as certain minimum benefit, etc., requirements are met, the plan can be any type of retirement
plan -- 401(a),
403(b), 457, etc. The rules which would apply to a FICA alternative plan, once the money was in it, would be identical to that of any other plan of a similar type. Thus, for example, a FICA alternative
403(b) plan could make a 90-24 transfer, if one was otherwise available, but a FICA alternative
401(a) plan could not.
Also, you have to look at the specific terms of the plan and contract involved. Rev. Rul. 90-24 merely governs the tax consequences of a transfer, if one is permitted. However, the plan document, applicable state or local law, or the contract involved may prohibit such transfers.