Someone else may be able to confirm for certain but I'd say yes - serials stopped at the end of 2004 and the XX wasn't discontinued until 2006. And early 4th gens still had the old-style box.
Its always possible it was a warranty pair at some point. The stuff that came out of the warranty department was all over the place. Some with W serials, some with no serial. Some with a proper serial.
I think Oakley must have kept a scrap yard of some sort from the broken xmetals and reused what they could to repair some other pairs, but also cast pairs specifically as warranty replacements.
For example I've got a pair of plasma XX, verified as genuine by HQ but without a serial. As much as we've all been able to piece together about the inner workings there are always outliers that slip through the cracks and don't fit the "standard" of what we think we know. A good example of that would be the trio of purple Juliets.
I have one, but box is pretty beat up, so I'm not sure if the box was added later of if they came together from the start. Frame is unserialized and frame, lenses, rubbers, and nosebridge are like brand new. Box is beat up and glasses are mint. That's why I'm not sure if box was added later or if it came with the glasses. .