Isn’t it the exact opposite? If it just says zone 6, it’s the old system because no a or b suffix.
If the number space got doubled zone 6 would still exist but mean something completely different now. You can’t tell if it’s the old or new system at all because the first numbers all got re-used but with different meaning.
"Old numbering or new numbering?"
"I don't know, just says zone 6"
If you reuse your numbering system, you have to specify which version you're using. Using an all-new numbering scheme eliminates that.