Here's a lead on Captain Mastain

Jesse Castleberry served with with Regiment C of Flippin's Battalion under Captain John G. Mastain from the state of Arkansas.
Jesse Castleberry was born in Georgia on 10 Feb 1818.
http://freepages.genealogy.rootsweb.ancestry.com/~jojr/branch/c-docs/sarah.htm

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Confederate Pension Records
Surname: Castleberry
Given Name: Jesse
Application Number: 4000
Widow Surname: Castleberry
Widow Given Name: Harriett
Company: C
Regiment: Flippin's Battalion
State Served From: Arkansas
Division: Infantry
Pension County: Bradley
Death Date: 7/17/1887
Application Year:
Widow Death Date:
Comments: widow applied 1906

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Henry D. Flippin, Captain, Company A, 1st Battalion, 46th Arkansas Militia, was elected Captain of Company “A”, 1st Battalion, 46th Regiment Arkansas Militia, Dallas County, March 2, 1861

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H. D. Flippin, Lt. Col., 1st Battalion Arkansas Reserve Cavalry

M317: Compiled Service Records of Confederate Soldiers Who Served in Organizations from the State of Arkansas - Misc.