Your stamp is from the edge of the watermark mat, where the paper manufacturer had their watermark: KPV (KONGL POSTVERKET). What you are seeing is the last 3 letters "KET".
These are considered "collectible" to some specialists, so it would be to your benefit to properly ID the stamp.
re: Sweden Definitive Types 1910-1919
Your stamp is from the edge of the watermark mat, where the paper manufacturer had their watermark: KPV (KONGL POSTVERKET). What you are seeing is the last 3 letters "KET".
These are considered "collectible" to some specialists, so it would be to your benefit to properly ID the stamp.