The Pokémon Base set had 6 separate printings... at some point in the reprinting, the copyright date changed from Original "C1999" to Final "C1999-2000). We call these cards "Original Base Set Printing". 

We have another listing "Final Base Set Printing". They have a very minor changes and on some the later printings some cards have a lighter overall color or on the 1st Ed stamp. Check out both.

The overall condition noted is lightly played although most are much better - we could only choose one condition. Prices reflect the condition in our opinion...

Some history - Bulbapedia isn't clear on the printing issue.
https://relentlessdragon.com/pokemon-card-game/identifying-early-pokemon-cards/