Beacon Mount
The mount pictured is a Federal "B" mount.
Federal's advertising stated--
" For the hearse-ambulance combination or other vehicles in part time emergency service. These models have a two piece bracket, the lower part matching and identical with Federal universal siren brackets. The light can be attached or removed in a matter of moments."
In 1955, the B base lights comanded a $7.50 premium over the standdard "A" base.