The exact pose is known on coins of Antoninus Pius and Marcus Aurelius. The legend certainly suggests that the mint statue in 190 AD was nude with hand over the head and leaning on a column. Certainly there is the possibility that the mint owned more than one statue of Apollo or that they chose to use a different pose for some other reason. The fact remains that a small percentage of statues that existed in Classical times remain. In some cases we have records of things now lost while some works survive that were never mentioned in literature.
Coins provide another evidence of what once existed.
Certainly some coin types bear original scenes but copying famous art was a common source of reverse types. As this is being written, the United States just released the New Jersey quarter bearing the famous scene of Washington crossing the Delaware. Some things never change. The reverse on this coin shows the satyr Marsyas who challenged Apollo to a music contest with the winner to do as he pleased with the loser. Apollo, the god of 'Good', defeated Marsyas in the contest and chose to have the satyr skinned alive.