Original LEE Filters 105mm Circular Polarizer screw-in is designed for the 100mm Foundation kit holder, 105mm Front holder Accessory ring is required to attach it - found here.
This filter can also be used on the LEE100 holder with the LEE100 105mm Polarizer Ring.
*Please note: Due to the thickness of this filter, a vignette will be present on some ultra wide angle lenses. Recommended for 28mm (full frame) or 15mm (cropped sensor) and above.
The Polarizer specifically designed for the LEE100 system can be found here.