These are hardcoded as they're the old extension templates for 1.0.2005.1. These used My as prefix. If you want another prefix, copy the templates used and change the prefix. the templates used are:
derivedEntityAdapter.template
derivedEntityFactoriesAdapter.template
entityUsingDerivedAdapter.template
inheritanceInfoProviderDerived.template
all stored in the shared templates folder. You should bind the copies in a new templatebindings file, and make that file overrule the existing template bindings by moving the templatebindings file to the top of the list at tab 2 in the generator config window