Thank you for the response Chris. Here is the side view. The only markings are the straight line etched seen in this photo and the roman numeral IV in the backside. The line and the number are not in-line with each other.
A little more info, This was my first and only experience with Polaroid type 59. It is rated at 80asa. Moments before, I had shot a sheet of type 55 rated at 50asa, but I always shoot it at 25. I had a meter reading for that shot of 30sec @ f6.3. Had I used the same method and shot the Type 59 at half the stated asa (ie 40) that would have given me 20sec @ f5.6 according to simply moving the asa dial on my Luna-pro. So I would say you had a pretty good guess at this as well. Yours may have been more educated than mine of course, being that mine was more like grabbing a number out of the sky.