I must have driven around that town 10 times trying to find the angle where the towers lined up just like in Grandpa's photo. In Grandpa's photo there are clearly two towers. Volterra has three. It wasn't until I drove around on some back roads on someones property that the towers seemed to line up. One tower covering another - making three look like two. I thought i'd found it. I took the photos and proceeded up the hill very happy and relieved. This meant I didn't check Sienna or Assisi as people had suggested. It wasn't until weeks later that I realised that it wasn't a match.
This was really exciting as I could literally follow Grandpa's movements. Out the front of his hotel. Then he went to a castle, then he was in a car taking photos out of the window. The missing Photo - slide 22 - being one of these. Then, one of the slides - slide 35 - which the countryside looks similar to the missing photo has 'Toledo' written on it.
If you know where any might be please let me know. Slide 15 which is a the hotel name 'Hotel Cordoba Palace' in it. After a quick map search I saw that this is definitely Spain. Cordoba being in the south. Then, just a few slides away Grandpa visited a castle or church and then took the missing photo out of a car window. Maybe if we can find the castle/church we'll know what direction he went.
If anyone knows this area in Spain or maybe knows someone who might please let me know. Photo 01 is surely here somewhere. Please email firstname.lastname@example.org with any thoughts or new clues on this one.
Then I discovered the tiny numbers on the slides that had helped so much with Photo 31. I decided to apply the same theory from that revelation to this photo. Basically, the numbers on the original slides are the shot number on the roll of film that Grandpa took and the date is the date he had them processed.