I am curious about the supposedly American source as I know several high end extremely knowledgable art glass dealers on both coasts & I yes the Romanian & Chinese signature ops are well known as they are in the manufacturing process, however I know of nothing that has surfaced about any U.S. op unless its a post manufacturer signature op that can somehow be cut into any previously manufactured blank to create a fake. As for anyone running a full scale production operation (1) if there was a U.S. production/blank/signature manufacturing op, news would have spread like wildfire over here & (2) 95% of any U.S. glass operation who could accomplish this not longer exists & Fenton was the last production operation who had the manpower, skills, equipment, etc. (and they would never do anything like this) unless its a small scale studio operation we do not know about & in our glass community its tough to hide this sort of activity.
Feel free to contact me privately if you wish if you have encountered anyone who knows of such an operation in the U.S.