Hello,
This has been a topic of discussion in the Olds community for many years. There is some really good information on 70-71 SX production numbers on this site in the "Production Number" tab in the left side navigation. Terry has done a great deal of research on this and is as knowledgeable as anyone. I consider the information posted on this site to be the most accurate to date.
Here is some of the info:
1970 Cutlass Supremes with Engine W32 852 Holiday Coupes & Convertibles
As you can see above, the 852 total includes BOTH coupes and convertibles AND DOES NOT include the last 6 weeks or so of the 1970 model year production (Jul-Aug). It appears from all the available production numbers that there was about 75 additional SX cars produced in this time period.
That makes ANY 70 W32 very rare. Although we may never know the exact breakdown of 70 W32 cars, I believe it is safe to assume the number of W32 SX convertibles is really low - maybe 10%. So about 85 or so. If you have one, consider yourself lucky!
I hope this helps. Kevin