The first time Bonded Superdense shows up by name that I can recall is in Striker, which is not strictly BT/CT.
The other important techological innovation that creeps in the back door is "controlling the strong nuclear force at ranges beyond the atomic", which forms the basis for nuclear damper tech as well as most so-called "Meson" tech (guns, shields, commo). This shows up starting in CT Book 4, and probably constitutes the "fourth" basic tech assumption in Traveller engineering.
The engineering requirements for heat resistance in weapons and power systems are quite distinct (and easier to meet) than those of unimaginably high-tensile-strength structural materials to be used for large-scale rigid structures, and therefore would logically constitute a separate technological innovation, likely included in the aforementioned "tabletop hot fusion". Here IRL, there is a growing abundance of materials engineering solutions for high-temp applications in the prototypes of tokamaks and weaponized lasers, but I've yet to see even preliminary research on, for example, crystallizing iron...
MDT and MPT (Magic Power Tech) carry with them certain basic assumptions about materials tech (indeed, the history of technology is the history of materials tech; hence the "Stone Age", the "Bronze Age", the "Iron Age", the "Silicon Age", etc.) but Bonded Superdense and its ilk represent an additional technological assumption beyond what's necesarily included in the basic background until CT Book 5 and the introduction of hulls exceeding 5Kdtons (ignoring for a moment the CT K'kree Alien Module which actually post-dates HG2). Thus, MMT, if embraced, would constitute a distinct "fifth" basic technological assumption in Traveller.
