Just Games, Inc. in Downers Grove, IL. Gottlieb tested a bunch of games there over the course of a year or so. Guardian, Tylz, Knightmare, curve ball, insector and wiz wars all tested there.
It was called "guardian" the first time it went on test. I think it still had the same name the second time. I only remember them testing it with the trackball, it would have sucked with a joystick.
Granted it was like 30 years ago but I'm pretty sure the cabinet was almost identical to Q*Bert. I don't recall what the artwork was on the cabinet and I'm doubtful there was any custom side art at all. The cabinets for all those test games were very similar.
A funny story that I'm pretty sure I posted on that mamezach blog about this game. One day while I was hanging out at Just Games (where my cousin was the manager back then), this woman from Gottlieb came by to check on the game and she was asking me all kinds of questions about what I liked about the games, etc. After talking to me for a little bit she said "when you get a little older you should go to work for a game company, you'd be perfect for it" or something to that effect. I just stared at her blankly while I was thinking that sounded awesome but wasn't likely to happen. Not to mention the 13 or 14 year old me never gave a thought to things like finding a job and growing up someday. Anyway fast forward about ten years and I end up going to work at Williams Bally/Midway, a job that just sort of fell in my lap and thus started a career in the game business that has spanned close to two decades now, complete with all kinds of ups and downs.