As seen here - https://aroespresso.com/products/grinder-setting-dial-for-eureka-mignon $36+shipping on their site.
Second picture is how this looks on my Mignon XL grinder (I have two of these- I'm selling the new in box one).
This product is designed specifically for the Eureka Mignon series grinders and is durable, reliable, and easy to use. It solves the problem of losing your reference point when switching between grind sizes. You can easily adjust the grind size between espresso, pour-over, and French press, and the numbered dial ensures that you can always return to your preferred setting with ease. No more wasting precious coffee to dial in the grind again.
The ARO Espresso™ Grinder Setting Dial and Adjustment Knob is a two-piece tool that is simple to install and will change the way you grind coffee in your Mignon. The grinder setting dial has 100 increments so you can dial in your grind size with precision. The geared grind adjustment knob is more ergonomic and easier to rotate than the stock knob. It uses a 3:1 gear ratio, which means three full rotations of the knob equate to one full rotation through all 100 increments on the dial. This lets you make extremely fine adjustments and get the most out of your grinder.