I got a used cyclocross frame. Had it over a week now, but only noticed now that the cantilever mounting point had these things stuck onto it. I'm not sure what they are and they are absent on the mounts on the fork. I was able to slip one right off, but the other one is stuck. I can twist it clockwise and counter-clockwise endlessly, but it's not screwed on.
In the photo, you can see me holding the one that slipped right off. What is it and how do I get it off? Thanks for any help. Definitely don't want to damage the frame in the process.