Wow! What type of enclosure is he in? What are your husbandry numbers like? It does look like a injury of some type. Are you feeding live prey? Maybe a scratch from the claws of the rat or mouse while it was struggling? Well, after looking more closely I think that is the normal extra flap of skin that is on all pythons. It becomes visible during swallowing when the jaws expand and move to accept the prey item.