I just added a very nice cobalt 731 Tillotson to my 731 collection. It's dark cobalt with some characteristic pitting on the glass surface. It has one small pinky nail flake on the side base. No internal cracks, typically found in these. And a nice milk glob in the dome, but very hard to see due to the dark color.
The story is these came out of the Sherman, a civil war ship at the bottom of the ocean. They were discovered by divers and the bulk were 731s and 728s with, I think about 14 cobalt 731 Tillotsons and the rest aqua. Many were covered with barnacles and some were destroyed since reportedly the ship burned before sinking. This is also the cause of so many of the cobalts being fractured.
If my story is incorrect, please add accurate details, preferably from anyone with first hand knowledge. I'm going by memory, which ain't what it used to be. Thanks, Jim, for helping me add this piece.