Those are the proprietary shifter mounts Shimano tried to impose on the industry around 1985ish. There are a bunch of threads about how to deal with these.

Be advised the threading on these is 4.5M, as opposed to 5M on standard mounts. Hopefully, you have your original bolts, since this size is almost impossible to source. If you're out of luck, you might have to re-tap the mount threads for 5M.

Since you only want to mount a stop, the easiest option is to modify the cable stops, not the frame mount. I had to do this on a 1985 Centurion Ironman to accommodate bar end shifters.

Doc Cannondale's thread on modifying and re-tapping the mount itself:

http://www.bikeforums.net/classic-vi...ter-style.html

Some threads & sites on modifying the stops (which basically involves removing material inside the stop to allow the circular mount to fit in and to file out a notch for the keyhole-looking tab):

http://www.bikeforums.net/classic-vi...-brazeons.html

https://simplicityvintagecycles.com/...aga-continues/