I wouldn't worry over a slightly different number, it's most likely a batch number, to keep track of which heads came from which pouring, factories that cast these keep a sample in case of any production problems, if it looks ok no doubt it will be.
The zetec heads are very common for developing a fissure crack, due to super heated water/antifreeze being majorly abrasive if the head gasket blows, but these can always be machined out if not driven repeatedly with blown HG, so may as well keep as spare.
tabetha