I had no idea there was such a thing as a Dura Ace stem. If Shimano is selling these, I imagine the volume is small enough that they wouldn't want to setup their own tooling.

Sorry, Sy, this wasn't current production. It's a quill stem from back in the day. They also made them for 600 Ultegra. They weren't uncommon back when quill stems were the norm. But the fact that Nitto was the OEM is still interesting.

Stems

I work for a company that makes one kind of product and in order to do that, does exactly one unique thing. Both engineering and production and test. Other than that one thing, the parts we get come from all over the place and we can make them too. Any machine shop could. You need to order forgings, send out for heat treat and coatings, packaging, lots of steps and none of them are unique. Whoever can do it with the least money or schedule depending on the earned value. Stovepiped companies that do everything in house are pretty unusual now.
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I suspect you meant "vertically-integrated", but I learned a new term today.