Could be a number of things..
First, pictures would help.
Are you sure it is a 1520 cal movement and not a just a 1520 auto bridge on a 1570 cal movement? Does the movement main plate have a serial number on it?
If it turns out to have a 1520 movement, it did not come from the factory that way. Either the auto bridge or movement could have been replaced at a service at some point.