It is kind of interesting. At one time I assumed the "W" referenced a magnetic plug since the 3/4 plugs on the muncies and later 67 and up 12 bolts were cast with the W also and they were magnetic as well as the 1" ones used on the 65/66 12 bolts but these I have are non magnetic bought for a 66 -L72 and are actually date coded also on the inside.One is march of 68 and the other Feb of 69.Strange why Gm would put that much detail into a plug??
