Having seen your “favourite historical document”, I find that sort of visualisation absolutely fascinating. Where can I read more about how they developed that model?

Unfortunately, I don’t think you can. 

That document was found in the out-sized exhibits section of the Committee on Economic Security’s papers, and I didn’t find anything else relating to it or its author Lewis Baxter or Economic Security Associates in the rest of the CES’ papers. The only other place I’ve found this Lewis Baxter mentioned anywhere is that he wrote an unsolicited amicus brief to the U.S Supreme Court on monetary policy in a case in the 40s (I think). 

I think that Lewis Baxter and his model are historical lacunae, where we just don’t have the sources because they didn’t survive to the present.