It's a minor variety and is due to uneven inking. You can see heavy parts elsewhere including the medallion background, plus what I'm guessing is a dry spot around "OF" rather than a scrape.
No premium really for this. A nice clean card with receiving postmark on the face is always nice to have. For all the hilariously poor quality eBay
auction items posted here at SCF, the eBay
covers of this vintage are often in pretty sad shape.