Anne & Paul, yes in all probability (high) its U.S. I have always been a bit suprised that close to this entire category of early milk glass production in the U.S. is lacking any solid documentation as to attribution while not nearly the case in Europe. This was produced for a decent period of time here & there were multiple manufacturers, Atterbury, Boston & Sandwich, Hobbs Brockunier, Dithridge, Challinor Taylor, Richardson Hartley, etc. who were the early manufacturers who evidently did not produce as many catalogs as did the European manufacturers so a high percentage of the category even in references is unknown or Possibly? Compared with EAPG manufacturers of the same time period, milk glass attributions fare poorly. Now the later manufacturers like Westmoreland, Imperial, Kemple, etc. hold their own because catalog were produced. If you had to pick a major "weak" category for U.S. glass production regarding product attribution, early milk glass would be my pick by a huge margin.
Here is a sample of a typical discussion between U.S. early MG collectors: "Well its certainly possible" "Well maybe?" "Do you think?" & finally "perhaps". It really has been like this for decades.