I used as my source one of Bryan's earlier postings to this board which laid out the numbering of the companies at three different times:

Mustered into state service as the 1st Arkansas, 10 Companies

Mustered into Confederate Service, (minus Yell Guards and Hindman Guards), 8 Companies

Reorganized as the 15th Arkansas, 8 Companies

It seems that the letter disgnations changed at each reorganization.

As for the Voluntee Companies from the 24th Militia Regiment of Jefferson County, I have the following

"Jefferson Guards" Captain Charles H. Carlton September 24, 1860
"Southern Guards" Captain Joseph W. Bocage December 18, 1860
"Pine Bluff Artillery" Captain Frederick Stick April 21, 1861
"Bradley Guards" Captain J. Bradley June 17, 1861
"McCullock Guards" Captain George W. Bayne July 24, 1861
"Arkansas Travellers" Captain R. M. Wallace July 24, 1861
"Hardee Guards" Captain James T. Armstrong August 2, 1861

The dates are the dates the officers were commissioned are are usually a couple of weeks after the date of the election:

Send me you email address and I will email you a copy of the List of Militia Officers for Jefferson County.