This is called a 'perfin' and is not damage. The SCF Glossary defines 'perfin' as "Stamps perforated through the face with identifying initials, designs or holes in coded positions. Perfins are normally used by a business or government office to discourage pilferage or misuse of stamps by employees. Perfins may be either privately or officially produced."
While some collectors avoid 'perfins' other hobbyists seek them out, especially on cover like yours. Don