Today I'll start with an insert called "Erasable Marker Memo" on the Web site and on the invoice, and "Most Used Telephone Numbers" on the package. It's a single sheet of plastic, about the same weight and flexibility as a dishwashing-liquid bottle. In other words, quite a bit thicker and stiffer than paper.
The insert is ©1995, and the packaging seems to be from the same era. (The more recent packaging is mostly white with only a little dark blue.) Also, it was made in the UK, which makes it a rare find these days.
The front of the form reads "Most Used Telephone Numbers" (as shown), and the back says "Messages," with just a page of blank lines below.
The instructions read, "Write in pencil or non-permanent marker. Erase with rubber eraser or damp cloth."
Just testing it briefly, pencil marks look lighter than on paper, and erase easily. Marker also appeared fainter, and rubbed off easily with just a finger -- no water needed! That leads me to believe just as I suspected -- that marker would smudge more easily than pencil.
I would love to use a form like this as a To-Do list for ad-hoc tasks, or capturing temporary notes. But it would also be great for its intended purpose, since the numbers we call most often change from time to time, as family and friends' phone numbers (or relationships) change from time to time.