For Social Security coverage purposes under section 218 of the Act, interstate instrumentalities are treated, to the extent practicable, as States, that is:

(a) They must be legally authorized to enter into an agreement with the Commissioner;

(b) They are subject to the same rules that are applied to the States;

(c) They may divide retirement systems and cover only the positions of members who want coverage; and

(d) They may provide coverage for firefighters and police officers in positions under a retirement system.