This is pretty typical of an early change for the 1967 cover as outlined in Bob Mannel's Small Block V8. On page 7-22, the C5OE-6059-A1 part number remained the same since 1965, but the part did change from ribs to no ribs early in 1967 and is depicted quite well. As you have noticed, the 1967 (late 66) cover you have has no ribs, which is indicative of the change for 1967. It is also mentioned that material was removed (excessive casting near the water outlet) along with the ribs during the change. I bet if you compare these pieces further, then you will notice the change there as well.