4400SS: This part fits the ORIGINAL 1-4400 Housing
If your housing is numbered 1N-4400 or 1P-4400 on the inside, OR has the slot on the dog post for the c-clip, you need the newer part number.
4500SS: This part fits the ORIGINAL 1-4500 Housing
If your housing is numbered 1N-4500 or 1P-4500 on the inside, OR has the slot on the dog post for the c-clip, you need the newer part number.
5500SS: This part fits the ORIGINAL 1-5500 Housing
If your housing is numbered 1P-5500 on the inside, OR has the slot on the dog post for the c-clip, you need the newer part number.
450SS and 550SS: This part is a substitute for the 10-550 Ratchet. This ratchet has less gear teeth, which creates a different amount of Anti-Reverse stops. It still functions as a suitable replacement.