This item is used on multiple models.
To be used with original 38-965 thumb screw
Comparing 965 reel versions:
Older 965 reels use the 38B-965 washer. Newer reels use wider diameter 38BN-965 washer.
Outer diameter on newer 38CN-965 is 3/16"
Outer diameter on older 38N-965 is 5/16", 1/16" smaller than the newer ring.
The above design change also appears to apply to the 975, 975LD size reels. See the comparison photo above.
This is the ordering screen for the smaller size 38C-965 original o'ring for the original 38-965 thumb screw