Japanese-made sewing machines from the 1950s. Most are all-metal and overbuilt, and will work like new with a few drops of oil, maybe a fresh belt. In the US they were imported and had local brand’s names put on them; what you’re really looking for is the “Made in Japan” on the back or bottom. Granny sewing machines also qualify, but most of the Japanese ones have zigzag
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There are also low grade threads/fabrics that consist of shorter individual fibers. They wear out faster.