I don't think the Riviera was earlier, Andy. The Cal Neva Lodge 70th chips were issued in 1996. Had the Riviera issued any L.E. prior to that? Not according to TCR.

I think the Cal Neva Lodge was probably the first stacking set issued in Nevada. The Circus Circus Reno set was certainly later. The only other contender I can think of is the Buck'N Ball set from Diamonds in Reno that was issued in 1996.