Possible.
GE introduced the Merry Midget light (what I've always called the rice bulb, but rice bulbs were smaller and less common) around 1965 and they became really popular REALLY quickly. They burned cooler, used a lot less electric, and were easier to hang on the tree because they weighed a lot less.
Many families I knew in the 1970s had largely switched to the Merry Midgets; my Grandparents did, as well. My family didn't. We liked our trees awash in light, and in the big rooms in our Victorian they really made our big trees stand out.
The C7 strings were probably available, but likely were relegated to a small section of the display simply because the Merry Midget style outsold them by a wide margin.
When LEDs Christmas lights started coming out larger bulb styles started to come back into style.
We finally stopped decorating with outdoor lights because there were a bunch of ahole kids in our town who thought that it was the height of great fun to go up to houses, unscrew the bulbs, and smash them on the porch. They were the same aholes who thought smashing pumpkins on cars was great fun, as well.
One guy caught one of the kids (he was 17 or 18 at the time, so not really a kid) smashing a pumpkin on his car and beat the hell out of him -- REALLY beat the hell out of him. Broke a couple of bones in his face, a couple of his ribs, I think his arm... really felony assault. Only, when the police arrived, the kid had a knife (never really clear whether the homeowner planted the blade or what), but it was enough that kid got arrested for attempted robbery, homeowner was backed up by a neighbor who supposedly saw the whole robbery attempt...
Since the kid already had an extensive juvenile sheet, you can imagine how it went down. Homeowner got off scott free, kid got some time in county jail and, oddly enough, a lot of petty crime and vandalism in town.... stopped. And most people in town were of the opinion that it couldn't have happened to a more deserving person.