That's industry tradition. It's not common to number or name launches, but rather to refer to the primary payload, e.g. “The MSL launch”. EDIT: I'm sure that t

That's industry tradition. It's not common to number or name launches, but rather to refer to the primary payload, e.g. “The MSL launch”. EDIT: I'm sure that the launch provider has some naming scheme referencing the rocket itself, but usage of that is almost universally confined to internal communications. Human spaceflight is perhaps the exception (STS-125, not Hubble Servicing Mission #4).