Folks,
I think you need to consider jets in a different way to the PBY.
In fact, go back to some of the old standard text books to see the explanation of why a straight wing piston engine aircraft has two cruise speed for a given HP setting, that is what the PBY is doing.
Have a look at a typical L/D curve for a jet, it is much "flatter", and yet we found an equivalent to the older aircraft, from practical experience.
Some years were to pass before a theoretical explanation for what we knew in practice happened, there wasd a "step".
What seems to happen is that the boundary layer adhesion and hence a large lump of the drag, differs at a given Mach No., depending on whether you accelerated to or decelerated to that cruising Mach.No.
I found the "step that wasn't a step" very evident on B707 ( both -300 & 320) and B747 SP/200/300, but not so on B767-200/300 of B747-400. Not a ETP standard of analysis, but there you go.