I more or less did the same net research you did, my friend @eb801 and I could not find evidence of the cartoon applied onto the "278". The fact that most (if not all) models of "278" featured at the net sport the cartoon, does not by default mean that this is correct! The fact that a pic exists, depicting the "278" without it, menas that if you do not apply it, your model will definitely be correct! Providing a 813NAS cartoon and wanting you to apply it onto the 278, does not mean that Trumpeter is by default correct! Of course, almost all modelers will be tempted to use it, that might being the explanation of all models seen at the net carry it.
Bottom line: based on the existing evidence, should I build the "278", I would suppress my big temptation and would not use it. But if you decide to use it, you may assume it had been applied at a later time...
Looking forward to see this beauty crossing the finish line!