Late 90s, like ~1998.
I remember them from 1999 because I recall ordering some and having them delivered to my place of work, and I remember having to collect them from the reception desk and the conversation with the girl at the desk who said "I'm not here to take deliveries of your personal items". I left there after 1999 and didn't have the bike they went on prior to that.
I just checked the reviews on MTBR and the earliest one is 1998 - so that might give you some rough guide.