Proto Traveller is not necessarily a time period – as High Guard came in 1979, before the accepted 1980 “cut-off” point – and not necessarily a “tight” set of rules. It is an attitude, a style if you will. It is Traveller in the spirit of the first three books of the original boxed set, as well as early adventures, JTAS articles, and supplements. That is – small ship, small setting, simple rules. I’d also argue that it has a focus on civilian or quasi-civilian (ex-military) play rather than strictly military settings and plots as suggested by the weapon list in Book 1 and as opposed to the mercenary unit play of Book 4, the huge-combatant naval play of Book 5, and the strict military fleet-building play of Trillion Credits Squadron.
This Proto-Traveller attitude consciously and intentionally resists the Three Creeps – Complexity Creep, Modifier Creep, and Scale Creep.
“Proto-Traveller” is a term used to describe a retro-look at the Classic Traveller line. It means playing only with Books 1-4, as well as Supplements 1-4, and Adventures 1-4 for setting inspiration. (Both the game and the setting changed over time.)
As Omer points out, it really isn’t just details about setting canon or rules. It’s a point of view about design philosophy and play style.
It’s a fascinating and well thought out piece, and well worth the read.