Read about it here on Wikipedia:

http://en.wikipedia.org/wiki/Mark_Twain_Riverboat

I remember this ride from the late 50's. It operated for a long time.

I'm not sure how the "Sally Unlimited" 1981 copyright fits in, though. Today a scan turns up a place in Sunrise Florida, which probably had nothing to do with the Sally on this game token.

Horn and Hardart (the Automat company) had a pizza chain called Mark Twain Playhouse, which is probably the other copyright mentioned on the token. Interestingly, Horn and Hardart owned the Royal Americana Hotel in Las Vegas for a while, so there could conceivable be a slot connection yet. The token pictured doesn't meet Nevada standards, though.