For historic reasons, a few product properties were also allowed on schema:Offer.These are now more confusing than useful, and I recommend to remove them from schema:Offer.

gtin12
gtin13
gtin14
gtin8
mpn
serialNumber

In some cases, we also have to polish the textual description of the properties, as in the case of serialNumbe (currently: "The serial number or any alphanumeric identifier of a particular product. When attached to an offer, it is a shortcut for the serial number of the product included in the offer.")

If there is agreement, I am happy to craft a pull request for that.

PS: This change might break a little bit of data that is out there, but I think the pain is worth it.