OK, aa62 - I've looked it up, and I wasn't 100% correct in my memory -but I was close.
Here is what I was (slightly mis-) remembering:
Hidden Fatigue Cracks Suspected in C130 Fire-Fighter Wing-Loss
The prototype C-130A fuselage was made of 7075-T6 alloy aluminum. The T6 temper gives high strength, but the fuselage cyclic tests quickly showed it is susceptible to fatigue, leading to hundreds of failures, three of which would have been catastrophic. Lockheed changed the fuselage skin to fatigue-tolerant 2024-T3 aluminum and redesigned the structure to reduce stress.
But they did not change the wing center box alloy, which was 7075-T6. The C-130B and original C-130E center boxes remained 7075-T6, but poor results in the C-130E fatigue test, in which the center box lower skin suffered the most damage, caused a change of material to more fatigue-tolerant 7075-T7531, as well as a redesign to reduce stress by 20%. The wing center section continues to be a life-limiting part for newer C-130s.