Hello,
Lap Joint Stub End flange is complex component consisting from two class components AT_STUB_END_LJ + AT_FLANGE_BACKING. Please add both classes to your specification.
There is a tool in Specification Generator (File > Component Manager). The Component Manager utility enables you to select one or more catalogs/specs and search these files for specific components.
I hope this will help.
Kind Regards,
Artiom