The 2707 was to have been the SST model that had never left the drawing board.
The 720 was 8 feet shorter than the 707-100 series and was usually distinguished from its counterpart by not having the VHF antenna atop the vertical fin and by having only one overwing emergency exit instead of the usual two. Nevertheless, there were some high density seating versions built with double overwing emergency exits; and some international versions included the VHF antenna atop the vertical fin. Also, some 720s had optional brake cooling fans.
There also was a 707-200 series of which only five editions were built for Braniff. Braniff was customer number 27, so its model designation was: B707-227.
And Qantas' 707-138s were special "long range" editions featuring additional fuel tanks.
The 707-400 series were powered by Rolls Royce Conways.