It's hard to rule out a later date like 1970s. You are not the only one with a 15 clone bought this late, but they are fewer and further between. I think Singers last model 15s were made in the very early 60s? The Japanese production began soon after WWII ended, and the white, pale blue, even pink turns up in the early 1950s. They were in production for decades. Yours could of course have been in storage for a couple of years before it was sold, but usually not many. Even if the production in Taiwan doesn't have the best reputation, I would not disregard it or give up on it; it should run and stitch problem free. I don't think there's any production record on these, and a more accurate dating is difficult.