Perhaps I'm mistaken, but it is my understanding that coo is only necessary if the part is foreign made. If it's made in USA, then coo label/stamp is not required.
This is mostly accurate. Only wholly imported items are required
to have COO labeling.
But it's not quite as simple as saying "If it's made in USA, then coo label/stamp is not required." There are lots of classes of products that aren't fully imported, but don't meet the "made in the USA" labeling requirements.
Long story short, items that are assembled in the US, partially processed in the US, made in the USA, etc., are not required to have COO labeling. Companies often choose to include the labeling, but they don't have to.