You are missing part of it - the part that lifts up with the machine when it's opened. Since there are lift mechanisms that attach to it, I expect you are missing the hardware. Since I've bought several machines still attached to that part of the cabinet, I expect that is what happened to yours. Someone removed it along with the machine instead of just taking out the machine.
I've got several of this style, Free, New Home, Goodrich and National. I haven't turned them over to see what the castors are like, and they've all got some differences. I have never seen a Singer in this style cabinet, their small parlor has one door on the front, among other differences.