Simplify the following text for a Grade 14 native English-speaking person.
Original: They may be either regular issues that just happen to be printed in small groups ( typical of many early stamps ) , or special issues often commemorating some event , such as a national anniversary , philatelic exhibition , or government program .
Simplified: They may be either regular issues that just happen to be printed in small groups , or special issues often commemorating some event , such as a national anniversary , philatelic exhibition , or government program .