This may be a long shot, but in a local thrift store there was a machine badged "Standard", leaf tension, but a rotary machine as opposed to a vibrating shuttle. It was a "treadle only" machine, and only $3, but I didn't buy it as it was missing the bobbin case. I looked on my Boye needle dispenser to see what needle a Standard used and it said Boye 2 1/2, which is the CC1221, which goes with the New Home/Free Westinghouse rotaries I have owned:
https://www.quiltingboard.com/vbulle...2-t312039.html
And this machine also
https://www.quiltingboard.com/vbulle...t-t314593.html
So I was going to buy the machine but it was gone when I returned to the store. My plan was to try a bobbin case from a machine I have that uses the Boye 2 1/2 (CC1221) needle.
Needlebar.org does not have a lot of information on Standard
http://needlebar.org/nbwiki/index.php?title=Standard
but it does show multiple bobbins for the rotary, so as I said, my plan may have been a long shot.