I've flown the classic and the 400 in two airlines. Both had the procedure, even though the classic was a freighter conversion. The stops are webbing straps, orange in colour and have a loop sewn at each end. The aircraft is de-pressurised, the L1 and L5 doors are opend with the strops in place which restain the door at about the third open position. One end loops around the door handle, the other end I can't remember. And I can't remember where they are stowed.
I remember being shown the strop and it's use with the comment it was a last ditch desparation procedure. It's probable that some airlines have decided not to carry the strops which would account for the lack of knowledge of the procedure amongst some experienced 747 pilots.