Ok a 1B bowl was fitted to BSA M20, some Ariel and some Panthers. It has a cross drilling on the brass bit below the bowl for a horizontal fuel elbow to be fitted. It has no angle.
A 1BM was fitted to some Ariels and also had the cross drilling but is on a 15 degree angle which is quite acute.
BUT in 1950 Amal made a bigger cross drilling for the arm running into the float bowl to let more fuel in, this type has a bigger screw plug on the end of the arm than the earlier pre 1950 type. To mark these they put an M for modified after the bowl numbers so a 1B became 1B/M sometimes the angled line is hard to see. If this bowl is not on an angle it will be a post 1950 1B bowl therefore marked 1B/M.
Best regards and keep safe.
John
A tedious job indeed, and a over complicated thing. I have assembled quite a few, but worth it in the end. Think those army workshops would have soon change them to the standard twist grip.
Your right Ian I should put pen to paper for the pre mono type carbs but consider the interest is dying as everyone over here anyway is playing with MK1 carbs on Triumph Bonneville.
Hi Ian will contact Henk and get the ball rolling. Will start with the various BSF and other threads used on the 276 type carburetter as I don't think they are readily available.
Regards John