how do i tell the year of my m-4 bayonet?

i recently bought an m4 bayonet (the one for the m1 carbine), and was wondering if there is any way to tell what year it was made. It has the black plastic handle, so i know its not one of the very first models. It has CASE stamped on it which i believe is the manufacturer, but im not sure

Well it appears that what you have is not a usual bayonet. Case was one of if not the original makers of the M-4, but they never made one with plastic handles. All theirs were made with stacked leather grips/handles. Plastic handles were introduced in 1956 and Case was not one of the manufacturers. However, many of the M-4's lost their leather handles due to rot while being used in the Pacific during the war and it's suspected that many of these had plastic handles installed by individuals after 1956 because there seem to be quite a few of them around.