It is my pleasure to help in anyway I can. There is two keywords on the box. Glass and Ovenproof. I tried to find info on when glass dishes became ovenproof and I turned up Fire King and Anchor Hocking from late 1940 and 50ties. Before that it was cast iron and heavy pottery used in ovens. So I want to say from that time. I can remember my moms refrigerator dishes you didn't dare use them around heat. I don't know your age but in the 30ties and 40ties when you purchased a new refigerator You got square glass dishes with it for veggies etc. Could it be possible Gramma got it for a wedding gift? Anyway keep the box in good condition, sometimes boxes are worth more than the item. Have a good day.