Plenty of 1sts and mashups! Some were still easy to find cheap as late as 2015, in the $1or $5 bins as raws to read, but still, if you want them to be Near Mint, better bring your "A" game! Many inspirational stories lay some groundwork for the future in movies, comics and over all the lore of the realm. Complete with character building and the fleshing out of certain characters. They tried to pick and choose who gets the continuation treatment, but this series has some 1st appearances as well!
They create, oh what is the word I'm looking for, oh ya, Suspense Tales! That has a ring to it, and put in the right order, Marvel makes something of it with 1sts and 2nds, staying ahead of the game!
"Tales" to me always implied one offs or a type of story with the spin that these were in the off days comparatively to the then main hero titles. They had their importance, and rather than make them all one-shot issues back then, it was meant to incorporate into the bigger main story title and with any hero available or those that would then "make the grade." Distribution back then was just as much about the sales of an issue, hence these tales were of the heroes type category and the characters that found themselves helping each other out towards common goals. Mostly some Iron Man and other soon Avengers, but in some cases, it did introduce characters that in themselves helped the main storyline. In the case of Peggy Carter there were soon to be love tales, while in the air of heroics and thrusts of the common good in the way the heroes became more dramatically driven.