Yep, it's a serious issue but as I said in my other comment, managing things like this is a relatively routine part of aircraft maintenance. I've been forwarded a preliminary risk assessment that puts the overall risk as "medium". The severity of the problem is obviously catastrophic but the probability of a failure occurring is so low ("extremely remote") that the overall airworthiness risk is not as high as one might think.

Every Hercules has a centre wing box, not just the new ones, and the "rainbow fitting"[0] is the part that attaches the centre wing box to the outer wing boxes. There is a rainbow fitting on the upper and lower surfaces of the box.

It looks like in this case the problem affects aircraft with more than 15000 equivalent baseline hours on their CW rainbow fittings, which is what's being inspected.

Source: am aerospace engineer with Hercules experience

[0] the rainbow fitting is a curved surface designed to transmit loads between two mating parts. It's called out as 99A in this pic: https://ibb.co/tbhcdS9