Look on the torque tube on the left side of the tractor for the engine serial number - it will start with either "BE" (116 cubic inch engine) or "CE" (125 cubic inch engine, also used in C and CA). The serial number may end in "G" (for higher compression, gasoline engine) or "K" (for lower compression, tractor fuel engine). That will help you get the right parts.
The tractor serial number is stamped beside the gearshift - I have an image of it on the website listed below. There are also some spec pages there. I have a 1949 and a 1950 B - you'll become addicted pretty fast!