The purpose of this type of Shaku-dokei is to simplify the adjustments required for variable hours and variable seasons on Japanese clocks. Here, a graph or Nama-gata is used, over which a metal bar descends carrying a time indicator. This indicator only requires moving every two weeks and by its position on the graph indicates the hour of the day for any season. Each nama-gata works only for limited latitudes because, as with the sun dial, it is based on the relative position of the sun. On the present example, the curve of the graph forms short spaces at the upper right and long spaces at the lower right - representing long days and short nights or vice versa, according to the summer or winter season.
For other examples with Nama-gata see N.H.N. Mody Japanese Clocks, Charles E Tuttle, 1967, Plate 84, Figs.1-3.