I seem to remember reading that the smaller venturi carb was sold as a replacement for Model A engines.
see the reprint of the B carb sheet on this page:
http://www.fordgarage.com/
click to open Carburetors, Intakes and Fuel Systems
then click Zenith Carburetors, Model B
a pic of an old B carb advertisement will show on the right side which explains the 2 venturis