This drawing illustrates the platen mechanism.  The top and bottom platens will be separted by springsin each corner.  When the springs are relaxed, i.e. no pressure exerted by the lever, the platens will leave a gap large enough to slide in the printing tray, plate, paper and blankets.  This gap needs to be carefully measured so that deeper plates may fit, for instance, metal type.  Thus it needs to be at least 1".  When the leaver is pulled, pressure is exerted, the top platen is forced down, compressing the springs in the corner.  The springs also serve to make sure that the top and bottom platens are always aligned.