It's quite difficult getting the facts as back in 1975 to 1990 at Nautilus it was a crazy period and things were slowly getting too big for Arthur to handle, he became pissed of with people ripping him off, suing him and in general he was disliking body builders more and more.
Casey was trained directly by Arthur, he went on to be the youngest ever mr America
Casey moved on because the weider circus had ostracized Arthur, Casey had a need to detach himself from him, move to California and trained with menzter.
Menzter and his brother did train under Arthur until he found out the menzter had been taking steroids against arthurs instructions, stories of Arthur beating up a bodybiulder behind the shed.
Arthur instructed security not to let him back in the place.
I feel the if Casey had stayed at nautilus he would have gotten even better, the steroids stuffed him up.