[Ok ... edited because I thought you were asking what the markings were. Sorry. I don't know the ans to your query.]
The receiver on my 1990 6601 is marked: Sporter Match HBAR - CAL. .223
The barrel is marked: C MP 5.56 NATO 1/7 HBAR
Despite what the receiver says it is chamber for 5.56.
I would guess a receiver marked 5.56 is likely a military/leo rifle.
BTW, my rifle has a large pivot pin. So it sure ain't .mil.