Southwest Airlines, the U.S. low-cost carrier which launched the original Boeing 737-300, 737-500, 737-700 Next Generation and 737 MAX has become the first customer for the shorter-bodied -7 version of Boeing’s re-engined MAX family.

The announcement brings total MAX orders to 1,315, including the recently confirmed deal with Turkish Airlines for 40 737-8s and 10 737-9s. It also marks the launch of the third variant of the MAX family, which Southwest officially inaugurated with a deal for 150 firm orders plus 150 options in December 2011. Boeing says the airline is converting 30 of these existing orders to the 737-7 version. As part of the deal, announced on May 15, Southwest is also exercising options for five additional 737-800s. With the 737-7 conversions and exercised options for 737-800s, Southwest’s unfilled orders consist of 180 737 MAX aircraft and 137 737-700/800s.

Until today’s announcement the 737-7, which is the smallest of the three variants, was the orphan of the family with no firm orders. The 737-8, in contrast, has taken 1,121 orders—or almost 90% of the announced business—while the stretched 737-9 has taken 164 orders to date. The 737-7 is designed to replace the 737-700 and, because of the lower fuel burn of its CFM Leap-1B engines and other aerodynamic and structural improvements, can fly up to 400 naut. mi. farther than the current model.

Southwest is expected to take its first 737-7 delivery in 2019.