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I also had this problem with my ~1990 Marin. To be honest, I have absolutely no idea how I solved it. Maybe just bought new brakes. And like you, I can't find any information on the internet about it - maybe @Hulsroy can help you, as a person who knows a lot about cantis/framebuilding/old frames
You could chop the posts off and tap whatever is left for a removable stud as linked above, I think @M_V came up with that idea.
Thanks - the posts are braze ons so not removable unfortunately but yeah finding different posts might have done it.