I don't know that Zone VI ever made available stickers that attested to the meter being modified by Zone VI unless the meter was modified. They did sell adhesive stickers that were used for zone placement...but those did not say anything about being modified by Zone VI. I have a modified Zone VI meter. Mine has the sticker that you mention.
Apart from that the only way that could determine the meter was modified is to disassemble the meter. I know that extra baffling and sharp cutting filters were installed in the modifications. But I would have a difficult time knowing what I was seeing unless I had an unmodified meter to compare it to.
Good luck, Donald Miller