I found this very nice article explaining the differences between the 707 variants. It gives a lot of detail about the different engines, wings, aircraft configurations and wich airline operated wich ones. Might be useful if someday someone decides to include different variants to FG's 707, that currently only has one (shame on me for not having the required modeling and programing skills, because this would be definitely something i would like to do)
https://airlinercafe.com/aviation-articles/ultimate-boeing-707-guide-84957/