I've been looking into this a bit. What type of flasher relay do you have. I believe original is NLA, 914-618-303-11, Hella 71373 TBB44 4DB 003425-02. Later superceded ones may not work the same, but I'm not sure yet.
https://www.early911sregistry.org/forums/showthread.php?171471-1972-911-Flasher-Unit