the slim line slots were intended to replace older mechanical slots. The standard bally cabinet was wider than the mechanicals, so some people had space problems.
however, they had to lower the handle in the slim lines, which took away hopper capacity. That wasn't a good thing for casinos - which typically didn't have cabinet width issues - so there's less slim line machines out there.
on my incomplete listing, only Harvey's Wagon Wheel and Cal-Neva Club casinos bought 855 machines that weren't generic bally games. 855-4 is a generic bally "lightning" machine.
I assume the game in your picture had a topper sign with the payouts ... my list says M-1433-152 is the part number, but I doubt that's very useful.
wrt parts you'd probably want to ask some of the vendors like KLAR directly.