Baby Lock Overlocker Upper Knife B4401-04A-OYBabylock upper knife for a range of Babylock overlockers. Part code B4401 04A OY Over time the effectiveness of the upper knives on any overlocker can become compromised resulting in difficulty cutting fabric or fabric bunching under the foot without giving a clean cut. Give your machine a new lease of life with a new upper knife. You can check the model number of your machine in one of three ways the front cover of the manual a sticker either on the