As someone who has worn quite a few 1960s/1970s era vintage clothes, I will say that a lot of the time the factory-made store-bought pieces are less "regular" appearing than such clothes today. The finishing, especially hems and such, was more frequently done by hand, even though it was still mass-manufactured and sold.
The Buckskin Girl's turtleneck looks to my eyes like a normal Sears-type purchase from about 1968-1972.
Some other posters are talking about clothing labels/lack thereof. It is very possible that BSG removed any tags herself, and if she did it carefully it would not be apparent (especially from photos). Personally, I consider clothing labels ugly and uncomfortable, so whenever I acquire a new item of clothing, I get out my embroidery scissors and carefully remove them by cutting the stitches (just trimming short the tags that are sewn into the seam or serging). When I do a good job, you have to look really closely to tell that a tag was ever sewn on.
Also keep in mind that side seam tags (with the fabric content and usually also country of origin and washing instructions) were not required on clothing back in 1981. BSG's clothes, if they had tags, would only have had neck tags. I don't know WHEN side seam tags became required, but I know that nothing I have ever bought older than about 30 years has ever had a side seam tag.
EDIT: I know the turtleneck had a size tag. I posted what I did because of the relevance to her jacket, as it was probably not handmade but also probably not sold as widely as some place like Sears.
ALSO: If BSG knew how to sew, she could have added the lining herself. As a second possibly relevant personal example, I have a store-bought leather jacket whose lining I have remade and replaced a total of 3 times over 12 years, because I love it so much. If that fringey jacket was BSG's favorite old piece of clothing (which I think is likely), she could pretty easily have done the same, and though it was store-bought, we wouldn't ever find its purple-lined brother.
