please take note of each of the muzzles:
the 7.62 variant is longer than the 5.56 variant. This is to prevent inadvertent “Sound suppressor” installation over the wron caliber barrel. Although you COULD screw the 7.62 suppressor over the 5.56 muzzle. But that's OK - it won't blow up in your face.
also, if you look just above the serration to the rear of the flash suppressor you'll see threading. That's where the “Noise suppressor” screw's onto the muzzle. Very sweet. You DON'T have to remove the existing flash suppressor.