BSA M20 has a 276C/1B carb with a replacement body of 276/014R. 276/402R were fitted to BSA B31and B32 350 OHV. But they are both one inch where the air goes in but the B31and B32 is one and one sixteen where it mounts onto the cylinder. Will do the job though. Jetblock 45 is right for a one inch jetblock. The throttle slide should be a 6/4 which means it has a 1/4 inch of the front, You need to measure it as a 3 1/2 will be a 32nd less although someone may have filed it to make it a 4. You start at number 2 position from the top but may need to move it to suit.