At the point of nine pages of this, my observation is that much has been made of whether these companies can force a buyer to return equipment purchased from them, not whether it's advantageous to the buyer to do so. Regardless of the reasons for wanting the return of the gear, it seems fairly obvious that the companies have no real incentive to genuinely chase the buyer around with threats of legal action, nor would they fail to compensate the buyer for a mistake made by someone else. Either path would be a public relations nightmare for such companies.
While I wholly understand why the OP posted this for the eevblog forum community to raise awareness of unusual activity, I don't see anything particularly scary about the correspondence we've seen here. It's obviously a form letter, and it's just as obvious that the equipment they want back isn't special in its own right but the serial number attached to it is. This prolonged speculation regarding potentially nefarious motivations (or "super-meters" available for purchase only by shadowy figures) is just the sort of embarrassment most companies would prefer to avoid. I think the wording of the form letter tends to obfuscate this, and I also believe it's intentional. Lawyers tend to just allege things, and broadly, to get stuff done; they don't have to offer details that could potentially put their client at a disadvantage in some way.
I'd just give it back. You'd likely be better off in the long run, as has been broadly hinted. The David vs. Goliath approach seems sort of over the top to me. I just don't see a credible "threat" here.