As far as I know, the abbreviations are based on the early findings of different compounts that could be isolated from the adrenal cortex. This was in the 1930s with Edward Kendall and Tadeus Reichstein as the leading scientists (hence also called Kendall's compounds). Compound B is corticosterone, cortisol ins compound F and cortison for instance is compound E. However I do not know whether the letters were randomly assigned to different compounds or in alphabetical order.