The Elgin Giant Watch Case Co. seems to have existed during the late 'teens and 1920s. However, evidence shows that it was part of the Illinois Watch Case Co.
Elgin Giant Watch Case Co.
In the late 'teens and 1920s, watch cases appeared, marketed by, and signed by, the Elgin Giant Watch Case Co. (example posted by grtnev). Ads at the time showed that the company made 18K and 14K gold cases in a variety of colors and shapes (as well as traditional cases). Additionally, documents as well as surviving cases show that gold-filled cases were also made under the Elgin Giant name. This marking example is inside of a gold-filled, 16-size case that has the appearance of one made during the late 'teens or 1920s.
The company seemed to be operated as a separate entity, as seen in this watch case company group ad. Nevertheless, Jerry Treiman has reported that:
In my 1927 edition of "The Jewelers' Index", issued by the Keystone Publishing Company, Philadelphia, the Elgin Giant Watch Case Company is listed separately from the Illinois Watch Case Co., but shares all of the same office addresses. Moreover, the Illinois Watch Case Co. entry includes a parenthetical notation - "(Also Elgin American Mfg. Co. and Elgin Giant Watch Case Co., Elgin, Ill.)", further indicating that all of these entities were under the same umbrella.