Why an external RAT on this early propeller plane at Charlotte?
Many older aircraft could be equipped with an optional external air driven generator, including, for example, the Luscombe 8A. Some air driven generator models were granted a Supplemental Type Certificate (STC) allowing them to be added to specific aircraft, such as the venerable J-3 Cub. The reason these were used is that, as you surmise, the engine is not equipped with any mechanism to drive an alternator or generator, i.e., there is no pulley, no gear mechanism, etc. Examples of those engines, still in use today, are the A-65, A-75 and original C-85. Newer designs of air driven generators are still available today, such as the BPE14 TURBO STAND-BY ALTERNATOR. The disadvantages include increased drag and exposure to ground obstacles, debris and of course, birds and bugs!