The letter-number combination forms an index to correspondence in use during the 19th century. Only a 19th-century document specialist would understand how to use the index and know which set of documents use this index. War Department clerks who created Confederate military service files knew these indexing systems far better that most. For that reason you're always better served to start with the soldier's military service file. Of course they made mistakes, but their files are much better than we would expect a U.S. agency to produce today.

There's no reason to expect that the document in question would provide any new information. If not found in the service file or something similar, you'd need special access to Confederate records (Record Group 109) in the National Archives, or hire a researcher who could search for it.