I have a (bad) habit of storing boxes of my gear in the attic and sometimes it pays off - like selling them on marketplaces with the box obviously gets more resale value.

But in this case, I would’ve thought your invoice from Amazon should’ve been enough. I’ve seen a lot of gear with no info like S/No written on them, which means it’s just benefit of doubt, or honor system when it comes to establishing whether something is in warranty.

And yes, it does seem like a deliberate attempt by them to reduce the number of repairs they service in warranty given how they denied your claim. Or at least an extremely concerning oversight