San Tan Tactical STT-15 Pillar Lower/Upper Receiver Set, Multi Caliber, Anodized Finish, Olive Drab Green STT15-COMBO-ANO-OD The STT-15 was born out of a desire to develop a true ambidextrous AR-15 pattern weapon system, built to the tightest possible tolerances. The full suite of ambidextrous controls comes standard on all Pillar AMBI receiver sets and lowers and allows for standard manual of arms from both the right- and left-handed operation of the weapon.