The ones I have seen have the bell located under the frame near one of the steps. I now on an Alco RSD-4 (long hood front) that the bell on the very front on the engineer's side hanging just under the frame right next to the steps.
Varied greatly from RR to RR. Most of the time they were mounted to the frame either near the steps as mentioned or near the fuel tank area. Some RRs even mounted them steam engine style from a bracket on the front of the long hood.