I have carb spares (as most old bike riders do), including a spare 276 carb body. So I dug it out, checked the threads (yeah, surprised aren't you) and got the poking wire and ultrasonic cleaner off the shelf.
Replacement 276 carb body being cleaned:-
I stripped the old 276 carb so all the settings are right in the new one. My 276 carb in bits:-
Replacing the brass carb block in the body I managed to split the bottom threads.....
Plan B - oversized screw into throttle stop on original carb body - recut the threads by forcing the screw in a bit at a time. Job done.
Bodged but working - welcome to the real world of old bikes.