Sorry, for the delay. I got really busy, really fast.
The answer to your question about the source is an American Antiquity article by Franklin Fenenga and Robert F. Heizer. 1941. Page134 -143. The article is entitled The origin and Authenticity of an atlatl and an atlatl dart from Lassen County, California. The article has the specs for a willow atlatl, and the dart remnant.