The five buttons were Ibanez's "preset" switching system. The three white buttons were on/off switches for the individual pickups, and the two black switches activated presets. I think the black switch closest to the neck activates whatever is set on the white switches, overriding the 5-way selector. The other black one must activate the bridge pickup only.
I've never really seen clear documentation on how the system works. I remember trying it once when these guitars first came out, and i couldn't make sense of it. Part of the problem is that Ibanez pickups from that era (in my opinion) didn't have terribly distinctive tones, making it a little more difficult to discern different tones from different positions.
By the way, i love the white gold/pearl nickel hardware on those Pro Line guitars!