This model raises a flag when the mailbox door is opened.  After you retrieve your mail from the mailbox, simply flip the flag down to reset it.  It requires a length of 1.8 mm stainless rod.  I used bicycle spokes because that's what I had on hand.   There are 3 parts: the spring/flag base (medium sized part with 2 bumps), the flag (largest , square piece), and the door arm (Skinny piece with 1 bump).  Print with PETG, install on your mailbox with 3M automotive trim tape.  It helps to put some CA on the PETG print before the tape.  You will also need 2 small eyescrews to go into the underside of the mailbox post.  Installation video is at yourube: DIY: Mailbox notification flag!