Hi Anne - first you need to know that Double Dutch is much harder to find than the regular Windmill bowls. Date of production? Well probably from around 1910 ish (maybe a year or two later) through to the 1920s.
You've described the spittoon shape perfectly as a cauldron. Strictly speaking, spittoons were made as functional items. However, the characteristic shaping (the pinch and flare) at the neck of a vase or on a bowl, has been done from time to time. I should add that such shaping is rare. On bowls it is very rare indeed. Vases too, also rare. Interestingly, Brockwitz and Riihimaki used that shaping on sugars and jardinieres from time to time as well. It makes for a very sought-after shape.