English: Souvenir cup from Hansboro, N.D. (USA), manufactured by Heisey c. 1900, of "opal" (uranium milk glass) Note [1]
Update: as per [2] this seems to more likely be "Ivorina Verde" (vs Opal) and Hansboro wasn't founded until 1905 so that's probably the earliest it could be from; still likely pre-WWI though. Confirmed uranium content by (1) characteristic green UV fluorescence typically seen in uranium glass and (2) radioactivity significantly above background (~500 vs <30 CPM) as measured with a Geiger counter.