You may be right. "Orange" was a very rare color but there's at least one original orange 356A in Southern California. 5711 isn't "signal orange" it's more of a pumpkin color. That '59 beside Willhoit's was somewhere between signal orange and pumpkin orange. It was close enough to fool me, maybe the light affected the color reproduction.
I looked through my photos and here's a 1958 [low-bow front towel bar and "wedge" turn signals] 356A at the 356 Club's Dana Point Concours:
I'm more confident that this one's really 5711 orange.