Nice. Regarding the error, there are a few things I would make sure are good before trying to decode any value shown on the display on a Williams. First gotta check the power supply voltages because if the power isn't right it will be flaky or not work. Then, I had all kinds of issues with my Joust. Sometimes it would not start up at all meaning no number shown, other times I had 131 errors or other errors, sometimes it would work but have strange artifacts on the screen during gameplay including left behind pixels or parts of things, and the high scores would get corrupted. Turned out it was a bad 40 pin ribbon cable.
If the power is good, I would try reseating the 40 pin ribbon cable. When removing be sure you pry it up with a small screwdriver. Trying to pull the cable by hand could break or damage it further. If it works after reseating, improves, or you see different results than before; I would at least replace the end of the ribbon cable that is removeable if not the entire cable.
Another thing is the wire connectors on the boards will go bad due to loose wires, broken connections, dirty pins, etc. Try reseating them. I wound up replacing most of the male connectors on my Joust. The new pins are square and make better contact than the old smaller round ones. That was the first thing I tried. Still having all the issues noted above, I then did the 40 pin ribbon cable and it has been rock solid since.