I could easily see the target number board being treated as three independent drums each with various bits bypassed so that they can spin continuously until someone breaks the circuit (which would be the actual random part). Once that happens, each drum will continue to step until they reach a valid digit.
The end result would be not too dissimilar to a slot machine, and as an all-mechanical solution would be easier to fabricate in the 1970's than making a computer pick a random number and automate electronics to match.
That seems quite plausible!
Or, Occam's Razor being what it is, they could have drawn a target number and a multiplier out of a hat with S&P watching and then set up the Solari to load those numbers in on cue.
/This is Bob Stewart we're talking about. Never add expenses where there don't need to be any
//Frankly I'm shocked he kicked in the extra $250 to bring it to $50,000
///"We can build an electronic solution nobody will see, or we can buy a deck of Bikes at the drugstore for two bucks."
