Key sentence. Before the late 1940s it was rare to see a "Confederate" flag even in the South. The reason I put "Confederate" in quotes is that what is usually thought of as the Confederate flag was never a Confederate flag at all. The Stars and Bars was the Confederate flag used until 1863. Then a SQUARE version of what is incorrectly thought of as the Confederate flag was used in the upper left hand corner of a white flag which was used until early 1865 when a red stripe along the right edge of the flag was added.
The incorrect rectangular version only appeared frequently only around 1948 with the formation of the Dixiecrat Party and especially in the 1950s following the Supreme Court desegregation decision when many Southern states added versions of the "Confederate" flag to their state flags.
Even the Klan almost never used the "Confederate" flag prior to 1950. If you look at photos of the big Klan march in Washington in the 1920s during their high point, you see a lot of American flags but NO Confederate flags.
The fact is that the "Confederate" flag is revised history. The Confederate Battle Flag upon which the "Confederate" flag is based was ALWAYS square, NOT rectangular.
The rectangular version of the ANV flag was used as the Confederate Navy jack, and also used by the army of Tennessee.