The Trig-Easy trigonometry aids were produced by E H Needham of Culver City California during World War 2. The US defence industries expanded greatly during the war, particularly in California, and must have needed to train many new machinists etc. I think Needham produced the Trig-Easy charts for this market.
I think my set of Trig-Easy charts is complete. It consists of the following
1. A case showing the charts were intended for engineers, machinists and others involved in mechanical engineering.
2. A plane trigonometry chart and an eight page folding pamphlet.
3. A compound angles chart and 144 page instruction book with illustrations, many worked examples and tables. The book was copyrighted in 1943 and revised and enlarged in 1945. This chart is marked as being sold by Eugene Dietzgen.
4. A spherical trigonometry chart and 16 page pamphlet marked ‘This pamphlet is written with the view of helping defence workers who have become rusty with trig problems….’