These ships were originally Cimarron class oilers, launched in 1939 for civilian use. They were acquired and commissioned by the U.S. Navy in 1940-41. Due to the shortage of MARAD type C3 ships for conversion to desperately needed escort carriers, it was decided in early 1942 to convert four oilers to escort carriers. The conversion took around six months.
https://en.wikipedia.org/wiki/Sangamon-class_escort_carrier#Overview
> These ships were originally Cimarron class oilers, launched in 1939 for civilian use. They were acquired and commissioned by the U.S. Navy in 1940-41. Due to the shortage of MARAD type C3 ships for conversion to desperately needed escort carriers, it was decided in early 1942 to convert four oilers to escort carriers. The conversion took around six months.
https://en.wikipedia.org/wiki/Sangamon-class_escort_carrier#Overview