The gong ("trolly bell") on the short hood? Also the bells (and the mounting brackets) on top of the short hood were not all the same, as they came off of various Steam Engines, and apparently had been sitting in the shops for more than 20 years by the time they were installed on the C420s.
516 has both a gong on the side and a recycled bell on top of the short hood.
I think I read somewhere that the original bells were under the running boards and would get clogged by high snow drifts, hence the need to experiment with different bells.