A note about Dungeons & Dragons and Chainmail ...
It was Chainmail that introduced my teen-aged friends and my teen-aged self first to D&D and thence to other RPGs. At first we used D&D more as a "quick & dirty" man-to-man combat system than a dungeon-crawling RPG. That did change over time.
The first GDW RPG we played was En Garde and again we emphasized the combat system over the other bits. Our use of Traveller went through the same stages.
It was either December of '77 or January of '78 and we were all playing various SPI folios at a friend's house. Tom Connelly came in with that Little Black Box with the Red Stripe we all know so well and passed out the Little Black Books within it to anyone who was interested.
"It's like D&D, but with guns," Tom said.
My first look at Traveller was LBB:3 Worlds and Adventures. The first thing I ever rolled up was an animal.
Initially, all we did was use the combat system in LBB:1 to run all sorts of WW2 Eastern Front small unit action. Within a few months, I distinctly remember a few guys running LBB:2's ship combat system and remarking it's mapless nature reminded me of AH's Jutland. I also remember someone borrowing the books, rolling up a subsector, and then pushing a free trader through it in a simplistic merchant/economics game.
We eventually drifted into running one-session adventures of the "Patron wants you to do X" variety. I can't remember much about them except that combat was always involved. The first "real", multi-session, adventure I remember playing was Across the Bright Face and that's only because my PC died one hex away from the starport! That had to have been in 1980 at the earliest.