Pics are worth a thousand words, but I think I know what it might be. I saw a Woma python at a local reptile shop a year or so ago and the poor thing had a massive lump on it's back. I asked one of the guys working there and was told it was the result from a rodent bite when a previous owner had the animal.

Just guessing but since you state that a previous owner mistreated it my guess would be rodent bites that healed up badly.