Black plastic diffusing screen. The diffusing screen clips to the motor base. The Trion 45 diffusing screen is used in the 707U, 707SM, 707TW, 707.
The circumference measures 20¾ inch.
Width 5/8 inch.
Instructions for installing the impeller with a 1/16" gap:
Trion Impeller Installation (1 page, 70k pdf).