This discussion is to reasonably clearly spell out the rule/setting combination known as Proto-Traveller. I'll keep this OP updated as the model gets refined.
Proto-Traveller is a small ship, low traffic Traveller Universe, typically focusing on civilian or quasi-civilian (ex-military) play rather than strictly military science-fiction. It tends to prefer the simpler rules over the more complex ones: basic chargen over advanced chargen; weapons from Book 1 over the ironmongery of Book 4; and so on.
Its default ruleset is Classic Traveller, the best edition of which is probably Starter Traveller. Supplements 1, 2, 3, 4, and 6 are often accepted, as are Adventures 1-3 and all of the Double Adventures. LBB4 (Mercenary), LBB6 (Scouts), Supplement 7, and Adventures 4, 10, and 12 are sometimes used. LBB5 (High Guard) is often not needed.
I can't remember if I ever posted on this thread or not, so sorry for the repeat.
Traveller, in black and white, within The Traveller Book, Starter Edition, and Book 0 (or so I recall) stated it was a sci-fi RPG. Having played a number of sci-fi RPGs (what few there were, most of the sci-fi games at the time were wargames), I initially dismissed it because of the vanilla flavor design and lack of art.
When I got the Starter Edition as a present and saw the graphics, I became enthused, and thought it would make great media in future years. We stuck with the generic "Imperium as an optional background" flavor for a long time because the game seemed best suited for that, despite its claims otherwise.
I now see Traveller as a law enforcement and geo-politics laboratory with a lot of thought experiments scripted out, some ever so vague, which had me puzzled for the longest time. The Argon Gambit should have tipped me off, but I guess I was a bit blinded by my enthusiasm for adventure in the stars.
Star Trek, when you decode it, is a police and hospital show, and I now see Traveller in that vein. I think SJGames GURPS Space is probably more in line with what Traveller claimed to be all those years ago, verse what it's become.
It's been an interesting experience. Given my parents own investigative background, and my martial arts instructor's background, I'm seeing Traveller in a new, perhaps more sober, light.
I think Traveller's current dynamic of proto-verse-space-opera should keep it going for years to come as a laboratory.
Keep out witting the bad guys. Good luck.