This is a pre-AT trigger. It has a three position safety, safe-bolt locked, safe - bolt unlocked and fire.
http://i40.tinypic.com/2r3cxgg.jpg
The safety engagement is adjustable using a 5/64 allen wrench. Its the rearmost screw in the trigger. Turn it in about a 1/32-1/16" of a turn at time until there isn't any trigger creep in either of the two safe positions. Make it too tight, the safety will be hard to operate. It a balancing act between working and working smoothly.
When your done put a drop of nail polish on it so it can't move out of adjustment.
Bill