back to 1962? There was some production of Biwa pearls starting that year, in Japan. Strack (2004) says they produced almost 2 tons of pearls that year. There was no other freshwater pearl culturing done for freshwater pearls until the 1980's or so,in China. Your pearls have a good luster and could be biwas, it is the clasp that makes me think mass produced and that didn't start happening until the 1990's and became a flood by 2000.
So perhaps they are Japanese cultured freshwater pearls instead of Chinese, but I know of no way to tell for sure, because the mussels used were the same species, I think. Perhaps others can comment.
I checked out ebay and could not find a comparable in a Biwa. The only ones I saw were overpriced and have poor luster, so I doubt they are Biwas at all. At least yours has a Biwa-like luster.
I hope you get them restrung with a sterling clasp. They aren't worth much as a resale, maybe $50.00.