Well, I must confess that even though the engine was NIB, I got if from an RCG member who had apparently opened the plastic bag holding the muffler bolts. The staple must have rattled around in the box falling into the engine (probably through the carb mount opening or the exhaust port).

Having caught the problem it's no real issue, anymore. But, I will always check "NIB" engines from now on (I always tear down used engines I buy).

David