Bob, I think you have it backwards. If the existing 30A circuit had a 30A receptacle on it, and a 50A receptacle was installed on the same circuit, the 50A receptacle would be in violation of 210.21(B)(3).
However, if the existing 30A circuit was going to be dedicated to this power supply, such that the 50A receptacle installed was a single receptacle on an individual branch circuit, then the 50A receptacle would be OK per 210.21(B)(1).
Being a dedicated 50A outlet on the 30A circuit would make the installation acceptable.
