NPS CWS&SS tells me that out of 187 Sam Williams this is the closest match:

Sam'l. C. Williams
Regiment Name General and Staff Officers, Corps, Division and Brigade Staffs, Non-com. Staffs and Bands, Enlisted Men, Staff Departments, C. S. A.
Side Confederate
Company
Soldier's Rank_In Maj.
Soldier's Rank_Out
Film Number M818 roll 26

Sam'l. R. Williams
Regiment Name General and Staff Officers, Corps, Division and Brigade Staffs, Non-com. Staffs and Bands, Enlisted Men, Staff Departments, C. S. A.
Side Confederate
Company
Soldier's Rank_In Asst. Surg.
Soldier's Rank_Out
Film Number M818 roll 26