How does a dial-a-down work?

Answer 1

Found elsewhere:

The 4 is permanently set in the middle of the box. There are three levers on the side of the box. Each lever controls a set of louvers with a 1, 2, or 3 on them. As the levers are pulled down, the louvered numbers are pulled behind openings to hide the numbers.

Source(s):15 years officiating football, spending many years as a head linesman.

Some say this is hard to envision, but I can imagine a box with a window on the front and layers of louvers inside that can be pulled up or down in front of the fixed "4" with levers on the side of the box. Makes sense to me, except how does it work front and back at the same time, with the 4 fixed in between what, six layers of louvers (1, 2, and 3 on each side, right)? Anyway, someone send a pic and a better explanation, please!