Thanks for all the info. I looked at some Rochester BC images on Google, and I think you've got it right - a later car carburator, with a 2 5/8 fitting rather than the 2 5/16 the oil baths have.
If thats the case, did the car version use a paper filter? (I'd rather paper than oil.) Anyone know what make and model I might look for? I think I remember from researching the engine that some Potiacs used the 261.
