Rwy in sight,
The B720 was a short range version of the B707-120, as I recall only significant difference was it had no centre wing tank, just the tanks in the wings outboard of the wing roots.
I don't think there was any obvious way of telling the difference just by external view.
As I mentioned in a previous post, some B720 (ex-TWA, I believe) had freon air conditioning. I never saw that on a 707, but that doesn't mean there weren't some built with freon.
When you were flying aeroplanes from a number of different original customers, the flight deck setups were often (always) very different, unlike the present day, where significant variations in a glass cockpit are more or less limited to the colour of the seat covers.
On those jobs, I found I needed to adapt real fast -- an interesting change, coming from an operator whose fleet was absolutely one standard.