Utterly bizarre.
With all the talk of AVA "death stems" the only vintage quill stems I've seen that had failed in a catastrophic way were 3TTT stems on which the handlebar clamp broke exactly where this one is hinged. The ones that broke were not hinged. I have seen two, and heard of several others. My first thought, on seeing these, was that someone tried to fix one that had broken. To think that the factory did this... Well, that's appalling. My advice would be: recycle that.
Not to discount your idea, as we have all seen some backyard work that is downright scary, but this seems a little too polished.
Maybe it was a "fitting" stem that would be provided to a 3t retailer to help sell the handlebars? You swap in and out the bars easily in the shop, on a trainer or something. Maybe it was never meant to be ridden? Would explain why there aren't more of them out there.
I'm making all of this up on a lazy monday morning, so grain of salt and all.