not flex, as it was not developed at that time, as I recall
If not the first to use flex .. Qantas' Wal Stack (lovely chap) was an early player with this game on the 707s. I recall a lecture at Uni (he was an Industry guest lecturer for the odd course) wherein he described the raised eyebrow response of a crew at the start of flex ops .. the pilots were a tad surprised to see the F/E push the throttles up to set climb power after T/O. Thereafter he specified climb power as the min flex for T/O.