Those particular perforations are known as perfins (PERForated INitials). They were usually used by a company on their postage to deter theft of company owned postage stamps.
The perforations may be initials, words, symbols...
For more info you can visit the following 2 perfin organizations:
http://www.perfins.org/http://www.angelfire.com/pr/perfinsoc/They are quite a lot of fun and can be very addictive!

k