Southwest with about 80 aircraft and Alaska with about 2 where the only two airlines in the USA that needed to have their aircraft inspected and were grounded till inspected....
The inspection program didn't cover all of the Classic 737s, only the most recent that Boeing had built. This was due to a change that Boeing had made in the manufacture of the Classic 737s, a change that Boeing thought would make the aircraft last 60,000 cycles before inspection. Instead this aircraft lasted 30,000 cycles. Southwest has 172 737-300s and 25 737-500s and only 78 737-300s required this inspection. It is sort of hard to plan to inspect for a failure when the aircraft is only halfway to the point where the manufacture said the failure may occur, and you have aircraft that are way older being inspected that show no sign of this failure.