Bosch OEM No.'s Vary, But Basically the 120-171 Has a Cast Iron Nose Cone and a Longer Field Case. The 120-171A Has an Aluminum Nose Cone and the Same Case Length as the 120-171. Finally, the 120-171B Has an Aluminum Nose Cone and a Shorter Case Length Than 120-171 or 120-171A. In Most Cases, the OEM No.'s Interchange in All Applications.