Reading the document in the link it seems some of the ‘non-conforming’ valves were made as recently as 2009 so, presumably, the current operators of Pegasus and Mercury engines already rely on recently manufactured small batches and it isn’t, or hasn’t been in the past, prohibitively expensive.

I suppose the real question is: what proportion of the recently manufactured small batches of exhaust valves turned-out to be ‘non-conforming’?