Terry, I got to thinking that I should probably mention that the company designators I listed are the original designations established when the 15th Arkansas was organized on January 2, 1862. The regimental CSRs at the National Archives are based on reconstructed muster rolls made up on October 6, 1862, after the regiment was exchanged. At that time, the 15th Arkansas was reorganized and the four "orphan" companies were returned to the regiment (following the breakup of the 40th Tennessee), and everyone got new company designations. So, the CSRs reflect the new company designators. Here is a matrix that may help --

(new) Co. A -- formerly Co. I, 40th Tennessee Infantry.
(new) Co. B -- formerly Co. D, 40th Tennessee Infantry.
(new) Co. C -- formerly (old) Co. C, 15th Arkansas Infantry.
(new) Co. D -- formerly Co. G, 40th Tennessee Infantry.
(new) Co. E -- formerly Co. F, 40th Tennessee Infantry.
(new) Co. F -- formerly (old) Co. B, 15th Arkansas Infantry.
(new) Co. G -- formerly (old) Co. C, 15th Arkansas Infantry.
(new) Co. H -- formerly (old) Co. E, 15th Arkansas Infantry.
(new) Co. I -- formerly (old) Co. D, 15th Arkansas Infantry.
(new) Co. K -- formerly (old) Co. F, 15th Arkansas Infantry.

These new designations became effective ten days later on October 16, 1862.

Here is how the commander of (old) Co. D explained it on October 6, 1862-- This Company left Camden, Ark. 10th Jany 1862. Marched from there to Gains Landing. Arrived Jany 18th, 1862. From there to Memphis by transports. The Regiment was ordered to Danville, Tenn. thence to Fort Heinman, then to Fort Henry, at which place the Muster Rolls, Clothing, Blankets &c were lost in the retreat on the 6th Feby 1862. Were at the Fort Donelson surrender & surrendered with that place on the 16th Feby 1862. This Roll is made from memorization. I believe it to be correct as could be presented under the circumstances.