Nan, I think it might be better to list it as:
"Unknown maker - probably European".
But keep in your head the things we have all suggested, until some day a catalog is found that provides conclusive proof.
Some items will almost certainly never be fully attributed. I've been chasing one pattern for years and years, it's turned into (for me) one of the Great Carnival Glass Mysteries. There's no stigma attached to a mystery piece - it just makes it that much more interesting!