Sewing machine mechanisms
Here is the underside of a Free #5 treadle from about 1913. It appears that the mechanism of Joe's 'long bobbin rotary' electrified Free is the same, or nearly the same as this this machine. Possibly the term first applied to this mechanism, rotoscillo (spelling?) was hard to explain, or for people to remember. Maybe Joe can repost his photo here for comparison.
Dianne in Colorado