I think what they meant when they said they may want the box returned in case he needs yet another one on warranty is that they'd want the /latest/ box returned. I doubt they'd ask for the very first one as well, if the new one broke down five years down the line for example. They can't honestly expect you to keep all your malfunctioning pieces - that's not how international companies operate, to the best of my knowledge.
Here's the deal from another side. He owns a box. It breaks. Rather than make him send it in like the rest of us they let him hang on to it. If he gets a replacement (through the mails, customs, mistakes, acts of God, theft or whatever) the old box
belongs to MF not him. They are just doing the courtesy of not having him mail it back (larger expense you know, check back for all the complaints on this issue before this policy). If it becomes multiple replacements (that happens a fair bit) they might want them back for any number of reason (lower per unit cost to ship, inspection to determine why they're going bad, who knows?).
The box in question is either assumed as replaced in which case it now belongs to MF and should not have been thrown out, or it belonged to the OP still and he shouldn't have thrown it out until he had the replacement or instructions to dispose of it. As you no doubt know, such instructions involve a bunch of questions, the destruction of the box (rip the screen out) and two photographs to confirm it all. Take your pick on the status of things, but in either event I see he's SOL.
It seems he has thrown out the only evidence the box ever really existed? I see no way to recover, except maybe to raise a stink here?
Unfortunate for sure, but I think that's the way it really works.
OF