If you’ve looked at other photos, people had pulled back the railings almost immediately to get to the people so I’m not sure I would read too much into the position of where it is.
If that was the explosive device, that railing would have been pushed back into the crowd, and no where near the mailbox to be pulled against it by rescuers. Even if it wasn't displaced, bending one of those barriers at that severe of an angle would take tremendous pressure, very unlikely to have happened if one or even three guys were yanking on the barrier.