The first type were the "teardrop" style, which were used up until 1968 (?)
Then in 1969 to 1970 the style with the arched rib on the side and the slanted rear and pointed front (I have these on my 150K 1970)
Then in 1971 to 1973, the "sculpted" top, with the more blunted front, and the back.
Last, the 1973 and on...the modern ones like you have. I had those on my 150M 1975.