This is a link to an old thread on Ar15.com. A poster by the name of Flyer had this to say and it may be the reason.
FYI, some Maadi's were imported in the early 1990's that had the FA Y stamped receiver with the FA hole drilled and then filled with a rivet. I had one of these for a couple of months and got a call from my friend who was the dealer that I pruchased the rifle from. He had been notified by the importer (Pars or Intrac, I can't remember) that these were considered machine guns by the ATF and had to be returned. I had to send it back to the importer and they sent me a new rifle without the rivet (Y stamp and no rivet). Unfortunately, the new rifle was not as nice as the old. I did however only send the rifle back and kept all of the accessories. So be cautious when buying a 1990's Maadi rifle and look for the Y stamp with a rivet. If you see one don't buy it. I am sure that if there are any still out there, ATF is still looking for them today.