This soldier is listed twice in the NPS database. I am not familiar with TN commands & consolidations. The 51st TN is not listed within the Order of Battle I have for Fort Donelson, but Gardner does mention Fort Donelson in his letter.

2nd/1st Lieutenant Uriah Gardner
Company E
51st Tennessee Infantry Regiment

2nd/3rd Lieutenant Urial Gardner
Company E
51st Consolidated Regiment Tennessee Infantry

Barry