Short history lesson on the Mak-90 third hole rifles.
What is called the "third hole" is a hole located slightly lower and forward of the trigger axis pin hole. this hole is where the axis pin goes that holds the auto sear to make it fulll auto. Without the auto sear, no full auto. So the BATF says third hole=machine gun.
At some point iin the early 90's some MAK-90s came into country with the evil third hole already drilled, but with a rivet put in place so you can't install the auto sear. Well it doesn't take a rocket scientist to realize how quick someone who is up to no good could get rid of the rivet and have a full auto good time. The BATF eventually realized the mistake and called back all third hole MAK-90's. Their logic is "once a machine gun, always a machine gun"
Many people turned their rifles in, some did not. Others simply didn't know about the recall. So there are still some out there floating about willfully, or ignorantly. Problem is, they are not legal to own. They are in the eyes of the BATF an unregistered Machine Gun (even with the rivet still in place). So best advice if you see one is run away, don not pass go, do not collect $200. Do not ever buy one. Even if the hole has been welded up, the BATF still considers the receiver an illegal machine gun.