I would have thought this particular type of item is not regarded as a consumable in the logistic chain.
As such even without serial numbers they should be able to tie down all the ones manufactured, and hence where they are in the logistics chain. Whether they be in stock somewhere, or flying around.
Another words Boeing has manufactured x and we have accounted for Y. If they are short the number on the aircraft only, then clearly its what we are looking for. Collating that data may take a bit hence its possible to be a bit coy until that has been done.