According to the strike point, strikes came out in 1992:
The first of these tokens were won from slot machines in the Reno/Tahoe area of Nevada in 1992.

but according to this site,
http://www.winning-slots.net/bonus-slot.php they say it was 1995:
But in 1995, all of that changed. One company, Anchor Gaming, released a game called Silver Strike

so which was it? thanks.