Did some preliminary research. The chart below gives the year, the sample size, the number of AABA songs in the sample and the percentage of AABA songs in the sample. Took awhile, and my ears need a rest, but it *is* real.

1920               36        4        11%
1921         36        0        0%
1922         36        6        17%
1923         36        4        11%
1924         49        9        18%
1925               36        9        25%
1926               69        18        26%
1927           52        19        37%
1928               72        31        43%
1929               57        30        53%
1930               47        27        57%
1931               36        20        56%
1932               39        27        69%
1933               36        23        64%
1934               42        29        69%