Jamie, strictly speaking, since an officer's commission depended on Congressional confirmation, no legal commission could have been done after the fall of Richmond. However, appointment alone (by the army commander or superior officer) was considered sufficient to give an officer rank. In the Cis-Mississippi, there was a staff officer who received his appointment to colonel 4-25-65. In the Trans-MS, Kirby Smith appointed generals as late as May 16, 1865, and colonels after your 4-19 date. If memory serves, the Army of Tennessee was reorganized 4-8 to 4-19 and several officers received colonels appointments in that time. My research has not gone below the rank of colonel.