Piston Combination Locks
These are the most compact locks you will ever find in Minecraft. How do I know? They are 3 blocks thick! This includes the buttons, a block to put the buttons on, and the 'wiring'.

I've previously uploaded a combination lock based on the design by AntVenom, which we both claimed to be compact, but now I realise it really wasn't. We've been going about it all wrong! Pistons will innovate future combination lock designs and leave redstone wiring redundant.

In this pack I've included 8 examples of varying sizes of this piston combination lock concept. They all (Except the concept design which is purely for demonstration) come with a "Sand" variant which uses sandstone instead of stone.

I've also created two designs, the Monolith and Nano, that are easily resettable and I recommend them for survival maps. There is more in-depth information on these designs in the README included in the download.

I plan to add more designs, which is why I haven't set the progress to 100%. The designs included are finished. Also, UCCL stands for Ultra-Compact Combination Lock. I realise now that PCL for Piston Combination Lock would be a better, more specific name but that would require a lot of renaming.

WARNING: Any designs with the "Sand" suffix requires Minecraft 1.4 because I've replaced the stone buttons with wooden ones for camouflage. Because I replaced blocks using MCEdit, it causes a temporary graphical glitch where wooden buttons appear to be wood plank blocks. Re-logging generally fixes this problem but it could be used to your advantage, to confuse other players. SethBlings Random Blocks MCEdit filter may have caused this but I'm not sure. When I next update this pack with new combination locks, I will have this problem fixed as well.

Additional Notes

Apologies to AntVenom for making your design redundant ;)