The problem is that GM does NOT have the data to determine how many SS 396 convertibles were built...
We know that there were 86,307 SS 396-equipped cars built, and 8,443 V8/Malibu convertibles (VIN 13667)… But there is no breakdown on how many of those SS396 options came on convertibles, how many on hardtops and how many on El Caminos...
Since the SS396 option was available on 346,406, but only added to 86,307 of those, that's about 25%... Estimate/extrapolate/guess that if convertibles followed the same statistical distribution, then 25% of those 8,443 convertibles would be SS396 convertibles, or about 2,104..
But its just a guess... 1966-1968, between 25% and 31% of all V8 convertibles were Supersports, so that's another "check" on the estimate...
And that's just the "SS convertible" part... even trickier to guess at how many convertibles were 4-speeds originally (or even 3-speed, because the manual 3-speed transmission was the base transmission).
But without documentation, there really isn't any way to prove its really a Supersport and not just a Malibu convertible with Supersport emblems added by a previous owner…