Older bikes had stops that would slip around the downtube as you describe and get tightened with a screw. Ask an LBS, a bike coop or look at bikes at a thrift store. My two 10-speeds both have them.
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"He who serves all, best serves himself" Jack London
Originally Posted by
Bjforrestal
I don't care if you are on a unicycle, as long as you're not using a motor to get places you get props from me. We're here to support each other. Share ideas, and motivate one another to actually keep doing it.