Flatwounds were considered a premium option in guitar strings, and were the standard for steel strings for bowed instruments (Since Thomastik introduced them earlier in the 20th century)
Steel core strings did exist in that era for bowed instruments. They were used mostly for student (not fullsize) instruments. A good violinist in that era used gut core strings (except for the E, which was plain steel) - those were the "standard" strings that you'd use on a "real" violin.