I don't know, but I am surprised no one has replied in some way or another. It isn't a baffle, I think, because our tank design shouldn't need one, and it isn't really the correct size or shape. It could be a support for the gauge sender unit or some such, although I have looked into my tank several times and not seen anything like it.
Suggest take a flexible inspection mirror and spark-proof flashlight and peer around inside the tank and see if there are any corresponding weld marks as a clue to where it might have been attached. In any case, I don't think I'd worry about it.
