I flew B707s years ago and there was also a stated minimum in-flight flaps up weight in the ops manual.
You remember well.
The B707 had a minimum in flight (departure) weight due to the possibility of fuel tank outlet un-porting, during rotation.
Minimum fuel for dispatch...40,000 pounds.
Bangladesh Bimin found out the hard way about this during a takeoff ex- Paya Lebar in the late seventies, just after rotation with minimal fuel in tanks (destination KUL)...three of four engines flamed out, and they ended up in the grass, just off the side of the runway.
