Ulf, I've never seen a model B carb that doesn't have a round accelerator pump. I have a 261 in my truck and have a holley/weber carb from Langdons stovebolt engine company. It runs good with that carb.
There is a bolter by the name of carb king, that may be able to help you. The only thing I can think of is maybe the carb is off of a 216 and had a smaller diameter pump.
