Once again, you don't have to change ANY parts if you are buying a surplus LE gun. It can have an extended mag and c-stock with all the original parts. It is already 922r compliant since it was surplused from a LE agency. State laws may apply though. Check your local laws.
If it was owned by a LE agency or officer for duty use then it doesn't need to have any parts changed. It has already complied with 922r and can be sold as surplus by the officer/agency without changing any parts. Happens all the time.