Another reference is a book I wrote about uniforms of the 2nd Seminole War a number of years ago. It is entitled "THe Men of Fort Foster: Enlisted Uniforms, Equipment and Artifacts of the United States Armed Forces, 1835-1842" (Pioneer Press, P.O. Box 684, Union City, TN 38281)and was published in 1996. I mentioned various versions of the M1833 Forage Cap - especially those used by militias. It might be of some use. Try Amazon.com or Pioneer Press in ordering the book. I believe Osprey did one on the U.S. Dragoons and therefore might mention the forage cap you are researching. I hope I've been helpful.
Alex de Quesada
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