I have a black plate 12AT7 with a "D" shaped getter on a single copper support rod. It has one unique feature with the black plates that I need help with identifying the manufacture. I believe it to be a RCA based on other RCA black plate tubes I own. The 12AT7 identifying marking near the top of the glass has the rectangular shape, with angular lines at the corners, similar to other RCA tubes I own.
There is one unique feature with the black plates that I need help identifying the manufacture. The plates have the "wing style" construction on the plates synonyms with RCA 12AT7's with one unique wrinkle. The narrow tapered end of each of the plates bends inward at a 90 degree angle. I have included a couple of images that might help.
Finally, I have checked a gazillion online images hoping to identify this tube, but nothing seems to show up. Again, my main question is, who is the manufacturer? And, is this some sort of rare or unusual tube?