I'm not 100% sure about this, but from what I've read a warranty replacement is unlikely to have been VIN stamped, and if they went to the trouble to stamp it with a true VIN it would be the VIN of the vehicle it was installed in (what other VIN could they/would they use?) There are some k-code engines out there with no VIN stamp, these are assumed to be Ford Service Replacements. So I'm guessing yours is likely to be a swap from another k-code.