I've never seen that one and it certainly doesn't look photoshopped.
It also doesn't look like a permanent part of the tank, there is nowhere for the wheels to go when the suspension is loaded. I bet it's either a material saving prototype of specific railcar or just something made up in unit. They had specific cars for Tigers, maybe they tried something similiar for IIIs and IVs. It looks like there's a chassis since theres a dark block running the length between the wheels.