The boom is always on the rider side of the leading edge. And on the first photo it is fully inflated to 6 psi. (on the second one not)

I had a quick try the other way round as I was not sure anymore, but that does not work with the blue ropes for attaching the boom. And you are probably right that it might prevent it from twisting when it fits snug - either because it just fits right on the 4m or digs in on the 7m