At the other end of the moving ropes are the counter weights. These are basically small wooden boxes with similar arrangements to terminate the ropes as at the other end (round bar and cleat). The boxes are filled with enough lead to counteract most of the mass of the covers, but only need to be about half the weight of the covers due to the pulley gain.
The weights are contained in twin 'lift shafts' made from Contiboard (white faced chipboard) which are about 5' tall; seen here with the front cover off.
The ropes go through the plate at the top where there are pulleys to turn them down the far side from this picture.