From "U.S. Infantry Weapons of World War II" by Bruce Canfield.
E1 Flame Thrower - A few were produced by Kincaid Company of New York.
M1 Flame Thrower- Produced by E.C. Brown Company of Rochester, NY.
M1A1 Flame Thrower-Also by E.C. Brown Company.
M1A1 Flame Thrower-Produced by Beattie Manufacturing Company of Little Falls, New Jersey.
E2 was developed by Standard Oil Company
M2-2 Flame Thrower-Produced by Beattie Company, E.C. Brown Company and also RlF. Sedgley, Inc. of Philadelphia, PA.
A total of 41,452 units were procured from January 1, 1942, through December 31, 1945. The majority of these were the M2-2 variant. I hope this helps....as I said Canfield's book has an excellent section on these.