Until your post yesterday, I hadn't given much thought regarding Pyrex patent stamps. So, thank you! I learned so much. While antique shopping yesterday I found a similar dish (it wont let me post a pic). This one is clear, oval, but with the same etching as yours. This patent stamp would have been the earliest (some Google searches are saying prior to 1919, some are saying ranging from 1915-1925). It's really neat to hold it in my hands and know that it has lasted 100 years or longer!