The purpose of this board is as follows:
On the 515/535-mainboard (535_I2c.PCB) there is a chip called 82B715. With this chip you can enhance the build-in I2c. Normally the distance between I2c-chips can be a few meters, with this special driver you can take a factor 10 so the distance can be 10 to 20 meter.

The measurements of this board are exactly the same as the LCD-board, so you can piggy-back the LCD-board on this board and drive a LCD-display with keyboard over a distance of 20 meters. There are two PCF8574 (not the PCF8574A, so there will not be any conflict with the two PCF8574's on the 515/535-mainboard). If you use the LCD-board/keyboard on the 515/535-mainboard, you will have 16 in-/output points from a distance of 20 meters of your mainboard.