It's Struck Through Grease. On a die everything is reversed compared to a coin. The devices are incused and sometimes they get filled with grease and crud so when the die strikes the planchet, it leaves a blank area where (in this case) the last digit of the date was plugged on the die.
You can help narrow down the range of years that it can be by looking at the designer's initials on the reverse. If it is a '91, it should have the same reverse that was used from '88-92. Otherwise, it is '93+.
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