In all the sources I have seen, Cow and Calf is described as an oath, or an agreement, not as a law. That is, it applies to the buyer/receiver who swore the oath or made the agreement.
If it is going to apply to anyone who might later find the book in an abandoned library or loot it in a wizard's war, it has to be written into the Peripheral Code. I thought Hibernia might have done that, but searching for «calf» Iar did not find anything in the book. Has anyone seen any reference to tribunal rulings on cow and calf?
What I find challenging is the old covenant. Typically the players take over a Winter covenant. In the library there are a lot of books a century or two old, but all the covenant members who were there at the time are long gone. Who know if the covenant has sworn the cow and calf and to whom?
Obviously, it is the narrative which matters, and at the end of the day we need an interpretation which is restrictive enough to make some tomes rare and worth going out of one's way (into a story) to get, and still permissive enough to allow a trading community encouraging magi to meet each other.