Twisted downtube shifter boss. I thought it could be unscrewed and I damaged it. I have a replacement part that came with a whole set of NOS shifters. Can it be replaced?
How do I efficiently shift gears on a road bike with downtube shifters? - Bicycles Stack Exchange
Use Bar-End or Downtube Shifters for Bike Touring - CyclingAbout.
Don't give up on the downtube shifters. : r/bicycling