To my eye, your grandparents Airstream could be a 1970, 1971, or 1972 31' Sovereign
International. These three years really had no exterior visual differences. Inside photos could be used to narrow down the years within this range as there were interior changes made during this time period.
I can further narrow down your search by observing that it appears that your grandparents had a state name added in blue letters between the trim lines rearward of the "International badge on the side of the trailer, though I cannot make out the name of the state. This was a common Airstream "customization" during the 1970s. Since they bought the trailer used, it might or might not be their home state. The trailer also had ZipDee awnings added on the curbside, street side, and over the front window (possibly over the rear window, too, but no photo to confirm). Unfortunately there is no way to use these details, nor the big red WBCCI numbers, to help you find the trailer, though they might help you confirm it should you happen to stumble upon it by accident.
About the only option you might have to find it would be if you could find an old registration certificate or other document that contained the serial number. Maybe then you could do a title search.