To open boxes numbered 1 through 500, the combination R2-C1 would work as follows:
1) Turn Left knob to the first letter or number given.
2) Turn right knob to the second letter or number given.
3) turn the dial right to open.
For boxes numbered 501 through 800, the combination 6-18-6 would work as follows:
1) Turn the dial left at least four times ending on first # of combintion.
2) Turn dial right once ending on second number of combination.
3) Turn dial left to third number of combination.
4) Turn dial right to open.