According to information found in Frazine K. Taylor's book "Researching African American genealogy in Alabama" the voter registers were created from the loyalty oaths. You may find on the oath his signature or his mark. Some of the counties did provide more information such as a birthdate and/or place of birth.

You can see what she says about the register/oath if you go to google books and search for her book.

Also you can see another example of the wording at the link below. It is midway the page

Also "There are several points that users should understand about the 1867 Voter Registration Records before using the database. 1) Because the local military authorities responsible for registering individuals in 1867 may have interpreted the Act (see History) regarding the creation of the records differently, a number of otherwise eligible citizens living in the county may not be reflected in the volumes. 2) Each volume has columns for the following information: Name, Race, County of residence, Precinct, Length of residence (in state, in county, in precinct), Book and page where the individual’s Loyalty Oath is recorded, Native country or state, and other remarks. However, not every entry includes each piece of information. 3) Many entries have the first names abbreviated." [source: http://www.archives.state.al.us/voterreg/index.cfm]