There are a number of ways to do this, there are OEM adapters, cnc adapters someone designed, taking a machine screw and bolting a pair of downtube cable stops on, and others.
I modified a pair of Shimano downtube cable stops and made a thru bolt to hold them on for my 83 centurion Pro-tour.
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72 Geoffery Butler, 72 Gugificatizion Witcomb, 72 Raleigh Gran Sport SS, 74 Raleigh GrandPrix dingle speed, 74 Raleigh international, 81 Centurion Pro-Tour, 85 Gazelle Primeur, 29rBMX, Surley Steamroller 650b