[watmough]

My reading of the diagram says that 5 different exposures were needed to establish a 5+1 level gray scale.

Note that each picture is scanned simultaneously, and contributes to whether 0 - 5 holes are made in the tape.

At the receiving end, the number of holes controls the brightness of the beam reconstructing the image.

[shrx]

This makes sense, but I guess you could still achieve the same effect with only one exposure + a configurable readout offset with a variable resistor or something along these lines?