Simplex pushbutton teardown!
Before we tear it apart, here's the back: the red tumblers represent the "active" numbers, they can be entered in any order. (1358 8513 3185, etc)
Now, gut shots! Back removed:
More insides, the metal piece at the top of the pic engages the tumblers when you turn the handle
Here's a closer look at the tumblers. The tumbler closest to the camera on the left is in the correct position, meaning it's not used in the combo, whereas the third one back on the left will need to be pressed to line up:
Tumblers. Active (red) on right, and inactive (blue) on the left.