I have learned that many times the rear door glass is from something else. I suggest that you measure diagonally from upper corner to lower corner, then the width top and bottom, and finally, the height at the end and in the middle. Carry a cloth tape measure with you whenever you are out along with the measurements, and you have a good chance of finding another car that has that glass. Once you know if the glass was used by another commercial chassis builder, it will give you a better chance of finding it. Presently, you have limited your search for S & S, however, Miller Meteor or Superior might have also used that rear glass. Also post the measurements here, and I will check my 1969 Miller Meteor glass to see if it is the same.