Interesting killer marking on stamp. Want to know more about postmarks? I learned a lot from the book listed on Amazon about Postmarks on Postcards (the information is good for covers too.) I bought the kindle edition and the illustrations are pretty good. The actual book would be nice too. There are chapters on identifying cancellations by machine or hand. The most interesting part was about killer bars or marks made on the stamps.Below is the link to Amazon for this book.
Postmarks on postcards: An illustrated guide to early 20th century U.S. postmarks