The space frame holds a missing display screen, and is both the output and input device. It outputs a render of a 3D object (Which I assume was probably just some wireframe) on the screen, and you can rotate the space frame around as an input to change the orientation of the render and view it from different angles.
I believe this is a rotation encoder that relays the orientation of a mounted monitor (not pictured) to update the virtual orientation of an object rendered on the screen.
There was a screen inside the Meccano contraption. I think that when you moved the Meccano frame, it would rotate the axis and the picture inside the screen would move at the same time, thus displaying different angles of the picture. Not 3d but fake perspective.
A bit like the illusion with the Wii remote and the targets moving in sync from Jonnhy Lee.
