The AT20 is the predecessor to the XTX20. We brought it back to the Apeks line up because it represents an excellent value. This is an ideal entry-level regulator that can spoil divers in a hurry. AT20 Predecessor to XTX20 Represents an Excellent Value Ideal Entry-Level Regulatorv Can Spoil Divers In-a-Hurry High-Performance Regulator Designed for Water Temps Above 50AdegF (10AdegC) Nitrox Ready Up to 40% O2 Out-of-the-Box First Stage: APEXS AT20 Balanced Diaphragm Design Consistent Performance from Start of Dive Until End of Dive LP Ports: 4 Port 3/8 UNF Thread HP Ports: 1 Port 7/16" UNF Thread Body: Marine Brass, w/Chrome Finish Second Stage: APEXS AT20 Pneumatically-Balanced Second Stage Smooth, Easy Breathing Large Purge Button, Easy to Operate w/Thick Gloves Integrated Venturi Control: Provides Performance Boost at Depth Prevents Unwanted Free-Flow at Surface Extremely Efficient Exhaust Valve Improves Breathing Performance Compact, Integrated Exhaust Tee