An English nobleman had an establishment in London and in several other cities including some on the continent. To communicate with his agents in these continental cities he purchased a number of cylinder lock boxes. He carefully set a different unlock code for the boxes sent to each city so that he could know which city a box had come from.

When he was away from London his trusted agent was not given the codes but was instead given a puzzle that had a number of different answers. Each answer would open one of the boxes.

This allowed our nobleman, when he was away from London, to trust his agent when he said he had received a message from a particular city and reduced the risk of anyone else opening a box.

You will find a variant of the puzzle that the nobleman used below.

I have printed below the alphabet, less I, in an Arial Font. To discover a code you should use a logical method to divide this alphabet into two groups. You should then continue to use other logical methods to divide each group into two more groups until one of the groups has just 6 characters. All the logical methods used should have the same general theme. This group should then be used to try to unlock one of the cylinders. If it does not then you should start again using different logical methods. If it unlocks one of the cylinders you may have won a prize and you may, of course, try to find another code.

I have been unable to calculate exactly how many different 6 letter groups there could be given the implict, now explicit, need to preserve alphabetical order in the groups but it is certainly less than 64,000,000. Anyone finding a way of working out the exact number wins my admiration.