I've been able to read the original series of Farnborough reports covering the Comet accidents and facinating reading they make. The one point that lept out at me that is rarely mentioned is that the cracks around the ADF and other window openings were occuring on the production line as evidenced by the number of stop drilled cracks, including one of the fatal cracks on Yoke Peter. It is obvious that the cracks propogated despite these measures, with some cracks having more than one hole.
The builders recognised that the material was prone to cracking and applied the industry standard preventative action whilst failing to recognise its ineffectivness and the subsequent consequences.