The "Z" means it is an inductive resistor plug and the end number is the gap it was gapped to when manufactured.
All else is the same.
As one should always check the spark plug gap before fitting, there is no need to be overly concerned about the last number so the only real difference is that one is an inductive resistor plug and the other isn't.
Inductive resistor plugs are less likely to cause interference to electronics on board and to late model outboard CDI ignitions.
The short answer is if they are working fine, use them, but make sure they are gapped correctly.