My case is the same one that Yuki sells. Or used to sell, to be precise. According to a few members, it's currently out of stock at Yukiwatch.
As far as I know, the Vietnamese fake SM300s first started showing up about 10-15 years ago. Look up "fake seamaster 300" and you should find a lot of materials about them. My case and dial shared several features with those old fakes (stubbies on the dials, coloration, &c.) but is different in many other respects (caseback does not say "CERTIFED", for one thing). So I really don't know where this case comes from. The safe betting is Vietnam or China!
Yep...found it and it is on backorder. He sells the entire watch, but with the common "flat bezel" case. Too bad nobody is casing these with the more authentic case because it's not something I could undertake myself. I would probably opt for the watch with the inferior case and ETA as a compromise given that the dial stubbies are now gone and the caseback has been addressed. Your franken is far superior due to your resourcefulness and patience. I have lesser amounts of both and will end up with a lesser watch because of it. Thanks for the reply!