The Finlandia / Harvia Part # F40-D Timer/Thermostat Knob is specifically designed for use with D-shaped shafts, providing a reliable and precise control interface for your sauna's timer or thermostat. This knob is compatible with the Blue and White series of Finlandia and Harvia control units, ensuring a secure fit and smooth operation. Its durable construction makes it an essential replacement part for maintaining the functionality and ease of use of your sauna controls.