Some more thoughts....
W. E. Woodruff, in his work “With the Light Guns 61-65” pp.99-102, describes the formation and disbanding of the temporary “Bull Battery” in the defence of Little Rock in September 1863. The battery was under the command of Woodruff and 1st Lieutenant Anderson Mills “of Trigg’s battery, on leave”.
Woodruff goes on to state “Mills was first lieutenant of Trigg’s battery, so assigned from Marshall’s. He was acting captain for the new battery from its inception until the surrender”. The “new battery” presumably refers to Trigg’s Battery but I still can’t establish for sure how “new” the battery was. I suspect September 1863 (this could possibly tie-in with the company being in parole camp) but I have no hard evidence.
With regard to Mills as acting captain, Mills’ CSR record (R. A. Mills Arkansas Miscellaneous) has the note “1st Lieut Comdg 9th Ark Batty. Trigg’s Battery.” Perhaps this ties in with Trigg's request for a leave of absence (four days maybe extending for longer?). The battery would still have been described as Trigg’s Battery even if he was no longer in immediate command and Mills hadn’t been promoted to captain.