(Source: Pentax Japan K-5II/s page, Credit: Blog reader "jaad75".)
Look at the above official diagram and illustration. For the "new" K-5II and K-5IIs, the first thing is that only the central AF point is equipped with the f/2.8 sensor, for the horizontal one. That f/2.8 sensor is unique for being the one and only one f/2.8 AF sensor that the camera has. However, this sensor will NOT work with
