jimbeetle:
So, as this has not yet been confirmed as part of AF 447 I have to go with HazelNuts' suggestion and for now assume it might be a part of an Ariane (there just happened to be a launch that included two boosters on July 1st).
The only thing I would question about this is: how many Ariane launches
have occured in the area? And why, until now, have no other discarded parts of this vessel ever wound up on a beach in Guyana?