This is the Replacement Control Knob for the System One 50 Series CPAP Machine.
The knob turns the CPAP on/off and used for adjusting settings.
Note that the Knob is medium gray in color and the sides are smooth and not ribbed. Sides are also straight up and down and not flared out.
Please see the image of the PR System One 50 Series CPAP that is on the product page for which this knob fits.
Return on Warranty Issues Only