I undestand the Planes of Fame Corsair is a restoration. However, I went to several sites and found A LOT of pictures showing the step on the starboard flap. The pictures I found appear to be older, possibly wartime photos. I didn't list all the links I found but there are tons.
This was discussed somewhere else (another site) not too long ago (I wish I had saved the picture) and one of the participants in the discussion provided an old picture of a 1a that had the clearest (in my opinion) proof of the step.
I am no Corsair expert but I find it interesting that there is evidence of the step and so much debate about it. Maybe it was an early production thing or whatever, but it exists.