The video below shows how the combination locking system works and the Steps following the video give details on the construction of the cabinet and the locking system.
Step 2: Some stages of cabinet construction
Step 3: Making the lock rod plates
Step 4: Making the lock rods
The combination of the lock rods and the lock rod plates on the drawers should, in theory, complete the locking system and present so many combinations that guessing the correct one (or two) would be pretty well impossible. However, I should mention that this is Generation 2 of this wooden combination locking system. The first cabinet had pretty well an identical system to this one in terms of the lock rods and the lock rod plates - but it didn't take long for son number one to figure out how to beat the system. By applying a light tugging pressure on the drawer handle as you slowly twist each lock rod, you can eventually "feel" as each of the rod flats line up with the lock rod plate openings. To overcome this deficiency in the current version of the cabinet, a hinged "lock rod cover" with a linkage to a pins that fit in a right angle groove on both sides of each drawer securely locks the drawer in place when the lock rod cover is open. With this setup you can't twist the rods and tug on the drawer to beat the system because when you can see the rods to twist them (cover Open) the drawers are locked in place by the pins. The video above or here: http://youtu.be/jXR1rI32I5A makes this easier to understand.
Step 5: Lock rod cover and linkage details
Step 6: Some other details and a question
I'm hoping that some Instructable viewer can figure out how many lock rod combinations are possible, besides the correct one (or two), for any given drawer. (I don't know)