The length of one side is stitched and tied to the recipient's address label. After handwriting on label, it would be taped over to secure to the envelope. The recipient would open the envelope by pulling up the label and pulling out the stitching from the envelope. The perforation allows for a relatively clean tear.
My friend Sheetal is working on her own stationery ideas, and we'll possibly start an etsy shop together. Will keep you posted.