How did the FAA and Boeing identify the 175 737 Classics?

How did the FAA and Boeing identify the 175 737 Classics?: "

With the coming of Tuesday's Federal Aviation Administration emergency airworthiness directive, there are still a few key questions left unanswered by the identification of the 175 737-300, -400s and -500s worldwide that will require intensive Eddy-current inspections of their fuselage lap-joints.

For quick catch up, a 737-300 made an emergency landing in Yuma, Arizona on April 1 after developing 5ft hole in the upper fuselage, which has since been traced to a pre-existing structural fatigue.

Only about 80 aircraft in the US are subject to the inspections, and almost all are the Southwest Airlines 737-300s that will all have completed inspection by late Tuesday.

So what of the other approximately 95 Classics around the world?

Boeing says the group of 175 was narrowed down by two criteria:

1. The airframes in question had to have 30,000 or more operational cycles.
   
2. Southwest says the airframes in question were "designed differently in the manufacturing process". Boeing confirms there are differences in the lap-joint design, and the specific configuration, says the airframer, was phased out as part of a blockpoint change during the 737 Classic's production run.

The number of aircraft with this design is significantly higher than the aircraft identified by the FAA and Boeing, though only 175 meet the criteria when paired with 30,000 or more cycles.

The specifics of that design configuration are yet undisclosed, though just how much information is shared publicly is up to Boeing and the FAA.

The natural question that will come along with these available facts is what prompted the different lap-joint design in the first place? And what's being done to ensure the aircraft with this older design along and fewer than 30,000 cycles are properly cared for just as the higher-cycle aircraft?

"