Zane, Take a look at http://www.airracinghistory.freeola.com/Powder%20Puff%20Derby.htm
Radial-powered biplanes were used in racing back then, and it certainly is a possibility for your photo of aircraft with "race numbers" on them. The first transcontinental air race for women in 1929 was called Women'S Air Derby or later AWTAR, All Women's Transcontinental Air Race. Will Rogers nicknamed it "Powder Puff Derby".
The modern Powder Puff Derby for women pilots started in 1947, but it wasn't always transcontinental, usually somewhat shorter with some different start and end cities in the years flown. I believe the women's air race was started by the 99s.
