P E A C E
The binary code of the counter IC is transformed to decimal code by those 4 inverters and the 10 times three AND gates.
The OR gates encode them for the 7-segment display.
The connections of the OR gates is free of choice according to the letters or digits you want to program.
The 1st top left OR gate is the 'a' of the 7-segment display, the 7th the 'f'.
Understanding this enables you to write almost everything you want.
Enjoy
