there is a repair manual for a S+ and a PE + poker machine available, it has the layout and wire colors for the switches, I doubt IGT changed the configuration for the Slant top version.
Basically the ground for the series circuit starts at the test switch, there should be a green wire on one side and a yellow/violet on the other side. (it is here you should hook one side of your meter leads. use an alligator clip or similar) the switches have three connections on them, common C, normally open NO and normally closed NC. the yellow/violet wire goes from the test switch to the common of HOLD 1, out the NC as a grey wire into the common of HOLD 3, out the NC as a grey/violet to the common of HOLD 4 out the NC as a grey to the common of Hold 5 out the NC as a grey/yellow into the common of HOLD 2 out the NC as a white wire to the plug and into the circuit board. a simple series circuit, put the other meter lead on the white wire, on the ohms scale (lowest setting) you should read no more than maybe 4/6 ohms. every time you press a switch it should be open (no ohms reading). each individual switch should read maybe 1 or 2 ohms across each terminal, Common to NO and common to NC. you should be able to fix your problem using these guide lines.
Hope this helps
Jim