If your shelix is about 12 years old, you may have the old version of the kn400 tip which has the same outer geometry but has a smaller hole and uses a smaller screw with 10/32 pitch (not the 8/32 pitch)

The OD is approx 6.15mm whereas the newer opening is 6.70mm.  The inserts are not interchangeable so call if you are unsure.

Size = 15mm  x 15 mm x 2.5 mm

Solid Carbide 

edges :  sharp points with edges with a 100mm radius  (place edge on table and you should see a little light on each end, they should rock slightly)

Plastic Case Includes  10 knives

Genuine Byrd Inserts from Byrd.

SHELIX Insert Replacement from Brett Byrd on Vimeo.