Dawg,
I understood your question was not regarding installing this type of receptacle, just an interest in what the thought was behind its use. If your rally interested in an answer try the Bryant website where you might be able to contact them and get an answer. Some companies enjoy trivia questions like that and sometimes you may even stump them due to their age.
Most of the old twistlock type cord connectors and plugs were like that with dual ratings. Caused confusion in plants with both 220 volt and 480 volt operation. Thanks to Nema ratings they are a thing of the past.
Good luck on finding an answer
Ron
