Legal Question in Business Law in California

Section 2501 of California's version of the Uniform Commercial Code covers this. It says (in relevant part) "Such identification can be made at any time and in any manner explicitly agreed to by the parties. In the absence of explicit agreement identification occurs (a) When the contract is made if it is for the sale of goods already existing and identified; (b) If the contract is for the sale of future goods other than those described in paragraph (c), when goods are shipped, marked or otherwise designated by the seller as goods to which the contract refers; (c) If the contract is for the sale of unborn young or future crops, when the crops are planted or otherwise become growing crops or the young are conceived."

So, I'd say in terms of your question, it can be either marking or shipping, whichever occurs first.

Please feel free to contact me with particulars if this answer isn't sufficient, or for examples from cases.