Here is the K'nex Combination Lock. This is made from Micro K'nex to keep it small, but could be up-scaled to Classic K'nex.
WARNING: DO NOTtry to lock your room or hide your government secrets with this. Your arch-nemesis will give it a quick yank and the missile codes will be gone. Or your brother will just steal a quarter from you. Either way, this model isn't very useful.
Here's how a lock works:
Next...... Remote transforming Iron Hide
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Step 1: Assembly
Based on where you leave out the peg on the second disc and how your number disc is positioned, everyone's combination will be different.
To find your combination and unlock:
Turn the disc until the front disc has its gap facing towards the grey connectors outside the lock. Check the number. Turn the number disc the other way until the second disc is in position. Check the number. Then turn the number disc the other way until the last disc is in position. Check the number. Now you have your combination. Push the grey connectors in to release the lock.