On September 14, 1982, a year before Williams died, Russ Jensen talked to him on the phone.
During the conversation, Williams said that he was currently designing video games for Stern and for an unnamed Japanese company.
That is interesting enough in its own right, but he also mentioned that he had designed a video game/pinball combination that he sold to Gottlieb.
The video game portion had a Rubik's Cube theme and he thought that Gottlieb was going to call the finished game The Cube or Paparazzi.
AFAIK none of Williams' video game creations were ever released. Gottlieb did release the video/pin combination Caveman but that doesn't sound anything like the game Williams' described.
The two things that really jump out at me are that he said the game "used mirrors" and that the pinball and video game portions were "fully integrated".
From the description, that sounds an awful lot like the Pinball 2000 concept that Williams developed in the late 90s (chronicled in the documentary Tilt).
For those who don't know, Harry Williams was one of the absolute legends of pinball.
He started in the coin-op industry in 1929 and over the years he designed pinball games for just about every major manufacturer. His two most famous innovations were probably the first tilt device and Contact (often credited as the first machine with electricity, sound, and a kicker - though there were actually other machines that used them first).
It's hard to pick just one story from his career, but how about the tilt device?
Harry's first game was 1932's Advance, a 10-ball game that featured a number of mechanical gates that could be opened and closed with a well-place shot. The game also included a visible coin chute to discourage the use of slugs. One day, Williams noticed a player in a nearby drugstore hitting the bottom of an Advance machine to make the gates open without hitting them with the ball. Furious, Williams pounded five sharp nails into the machine to discourage such tactics. Knowing that he needed a more practical (and humane) solution he created a device consisting of a steel ball mounted atop a pedestal. If the machine was jarred too violently, the ball would fall off the pedestal and make contact with a metal ring, closing a circuit and bringing the game to a premature end. He called the device a "stool pigeon". After returning the machine back to the drugstore, one of the players activated the device and said "Oh look, I hit it and it tilted. " Williams quickly renamed his creation the tilt device.