I have researched and researched this as my husband was coveting the Hermes GM agenda for his birthday, and I have just returned from the store with the purchase of the agenda and an unlined notepad. The pen will have to wait until Christmas. There are 2 pens that will definitely fit the very small loops of the Hermes PM & GM agenda, other than the gorgeous sterling silver Hermes pen which is $200. I must say that the unique thing about the Hermes pen is that the base of the pen fits in the bottom loop so that it is a "receptacle" for the writing portion, which is beautiful. However, if you want to go another route, you can use a Cross MicroPen PDA stylus/retractable pen (about $30 and refill cartridges are a few dollars). The orange micro pen is perfect with the Hermes orange GM agenda; not sure if it is too long for the PM. The other alternative is the very pretty sterling silver "the wallet pen" as seen on Oprah as one of her favorite things. Website is thewalletpen.com. This is about $50, refills are a few bucks. This is super tiny and will work with both the PM & the GM.