The lock ring is important because it keeps the U-joint in place and keeps it from moving. If you have the wrong lock ring then the U-joint could rock back and forth and cause damage to the the yoke. If you have the outside lock ring it will have a circular shape with tangs and be at the top. If you have the inside lock ring then it'll look like a C and be at the bottom.
For those of you lucky ones with a 1972 model, you will need to measure the outer diameter of your bearing to ensure you purchase the correct u-joint. If you have a 1-1/16th outer diameter you have the early 72 model and will need a different u-joint than the later 72 models.