Lots of speculation of the years, but here's the real production # for L78 convertibles, from GM records.
1967 113
1968 162
1969 189 (thru Sept 69)
If you had multiplied the convertible percentage for each year by the L78 volume, those #'s would be 130, 398, and 276. Interest that the 68 guess would have been off 2.5 times, while 67 and 69 were close.