EntityBase2 implements ISerializable because it has an event. As soon as a class has an event, you can't simply put [Serializable] on a class' definition, you have to implement ISerializable, which is the reason why EntityFields2 doesnt implement it. Adding a class member gets serialized into the class automatically in the case of EntityFields2.
Word of warning: keep track of what you change in the ORMSupportclasses. The upcoming 1.0.2005.1 upgrade has some things changed in several classes, and with every upgrade you have to make sure your changes are re-applied to the new sourcecode.