The Rochester B is a manual choke and will generally be found on pickups with a 6 cyl. engine.
The Rochester BC is an automatic choke, also referred to as a hot air choke. It is a hot air choke because hot air from the exhaust manifold is brought up through a hot air tube to the choke thermostat. The movement of this hot air is facilitated by a vacuum draw from the carburetor. This design is usually found on passenger cars w/ a 6 cyl engine.
The Rochester BV type of carburetor includes a vacum break, or choke pull off. This is used to pull the choke off when under load and the engine is still cold.
Rochester B, BC & BV carburetors are identified by a tag attached to the top of the carburetor. Look for a number that starts with 7. Example 702140.
Some Rochester B, BC, BV carburetors had the last part of the carburetor number stamped on the bottom of the mounting flange. There will be either 4, or 5 numbers. In this case there are 5 numbers, 23105. Put 70 in front of the number, 7023105. Had there been only 4 numbers (2310), then 700 would be added to the number as in: 7002310
All other numbers stamped on the carburetor are nothing more than casting numbers for the part it is stamped on and in most cases isn’t enough to determine which parts to use.