I was buying these avidly around 1976/1977 and maybe 1978 at maybe two convenience stores along M-89 in SW Michigan. I do not remember ever seeing a pack that had a Marvel and a DC in the same pack. Just one or the other. I remember most being excited about getting as many Our Army at Wars as I could. The new issues were on issue 300=305 or so and you could get ones in the late 290s at the stores. My best find was getting one of the new DC dollar comics in a pack. So yeah, getting a giant was a big deal. I think I got a giant HOM later too.
As you guys say, these packs set up the interesting dynamic where the interior is a vivid childhood memory but when you add a copy with the cover as an adult, the cover feels a bit detached from the experience and you don't have the attachment to the cover. Kind of like when you have one part of a story as a kid and get the other years later. I loved Detective 447 to death as a kid. Reread it tons of times. When I got 448 years later I wasn't so interested in the rest of the story. Just another comic.
If the Mile High 2 collection is an affidavit return, which it is, of course, then that distributor must have been allowed to do affidavits far before the 1980s since the collection has Marvels back to what, 1966 or earlier?