It's mounted on a wall with a backing.
Naturally I was intrigued by them and decided to have a go at it.
I'll cut to the punchline and if any cares about the process I will post more info.
So , the "OPEN" tab is slid down when you have the correct combo punched in to retract the bolt at the top of the lock box. The OPEN tab will move even if you don't have the correct combo, but the bolt will not retract.
The OPEN tab is mechanically linked to the RESET tab, so when you slide the OPEN tab down the RESET tab slides down as well. Like so:
This was the locks undoing.
So how it works:
Slide down the OPEN tab as far is it will go, keep your eye fixed on the RESET tab. Push and hold a combo number to test it. It will not depress all the way, but hold it as far as it will go. Release the OPEN tab and note any change in position of the RESET tab. On a number that IS NOT used in the correct combination the RESET tab will not retract upwards at all. The RESET tab may move left or right if you switch which bank of numbers you are testing (left side to right side, or right side to left side). On a number that IS used in the correct combination the RESET tab will retract upwards slightly.
In this example, number 2 and number 5 indicated, as well as 2 other combo numbers. That narrowed down 16 combos, EDIT: I was just playing with this lock some more and found out ANY ORDER will work as well, as long as the correct 4 buttons are pushed.
Mechanical Locks FTW. I searched these when I got home and found that: "This strong box design has a solid track record for unmatched quality and security."