Quoted:
A quick, safe, easy way to fix this would be to pull out the all the FCG components, throw them away, and replace it all with new. I only mention this because there was a member of another forum who ended up doing a year in prison because his gun he bought used went FA and ended up having an M16 part in it. If there is no way to tell what the guy did or what parts he used in his build, or how to reach him, it is worth the $50 bucks or so just to get a new, safe, legal trigger group and have some degree of certainty what is in the gun. JMHO, but in this case it is a small price to pay to remove any and all doubt.
Why in the world throw out the FCG? Just get a disconnector spring, install it, and enjoy the rifle.
After looking at the pics the OP posted, there are no M16 parts in that FCG. Maybe, the disconnector, but the FA tail has been cut off. I say that because the rear of the disconnector looks a little uneven. A Bushmaster of mine has a FCG that, from the factory, was modified from M16 to AR15 (the hammer and disconnector––the trigger is AR15). It's really easy to tell that they were modified––the disconnector wasn't even refinished, heh. I don't lose any sleep over it.