I'm going to put my faith in the final Collection 2 501st figure using the SA body, and not
the Gunner body. I went looking back at the SDCC Q&A shots, and the same images that are being circulated now were used in their ppt slides -- here's the C2 slide with the 501st:
It's the same studio shot, Gunner body. That means those shots were done some time before July, and at the time of the show, according to Hasbro, none of the post-56 had started their runs. So, I'd wager that what was shown on the floor was closer to final than what was used in the studio shots weeks prior.
At least that's my hope.