Face it: most Traveller PCs would come from the Travelling social class. In LBB1, 4 out of the 6 careers are clearly Travellers - Marines, Navy, Scouts and Merchants; Planetary Army men are probably going to be more likely (except for the planetary navy and marines, that it) to access offworld technology than most other people on their world; and even many variants of Other won't preclude interstellar travel. In S4: Citizens of the Imperium, only 2 of 12 careers are directly space-related (Pirates and Belters), but 3 more are careers that grant starships as well (Nobles, Scientists and Hunters) - which implies that these careers are often space-related; 2 more careers are military (Sailors and Flyers) and thus more likely to encounter off-world technology; Diplomats, Doctors and Rogues also have a good chance of coming into contact with extraplanetary technology; and while Bureaucrats would have a lesser chance than most other careers to be travellers or access offworld technology, they are still part of an organized society, which, if in contact with the Imperium, will, other the centuries, acquire a certain level of familiarity with Mainstream Imperial technology. Only the Barbarians completely stand outside the sphere of Travellers.
In short, in the Classic Era, most Traveller PCs would be likely to be familiar with Mainstream Imperial technology. The main exceptions will be:
1) Characters hailing from interdicted or isolated worlds (Red Zones or outside explored space).
2) Characters hailing from worlds with Starport-X.
3) Barbarian characters.
Mainstream Imperial is TL9-14, though TL15 societies within the imperium is seriously influenced by it as well. Despite the seemingly large range, many concepts, ideas, design elements, interfaces, standards and, in some cases, parts are shared between these TLs. LBB2 seems to hint at this with its standardlized components, designed at TL15 but, in some cases, adapted to TL9 (or TL10-14) production.
