early ar15 question

I have an early Colt AR 15 made in 1964. It is marked .223 cal. Can 5.56 GI ammo be fired in it? I thought 223 ammo can be fired in 5.56 chambers but not the other way around. Thanks in advance for help.

I shot white box (20rd box)5.56mm Remington out of my four digit SP1 way back when I bought it. Do you have a three or four digit SP1 ? was it changed around any.

I understand the Colt SP1 was made for 5,56mm but they used the 223 Remington on the manual etc

There was a rare export SP1 in 222 Remington cal too

It is a 3 digit numbered .

This one is still new and really doubt it has been fired since the factoryAttachment 59366Attachment 59367Attachment 59368Attachment 59369Attachment 59370