Joe , at some point in its life it’s had a change of carb , the b10 is not the original carb fitted on a Bantam. That’s not to say that it will not run or perform perfectly OK.
The correct carb for your model is an S12 , which requires the correct manifold. It’s entirely up to you if you want to keep it all original.

Andy.

Joe , the b10 carb was used on the MK10/12 and 15 , and was used on a whole host of machines , so far more common , and hence easier to source parts.

Andy.