I must admit when I first saw this in the Carriage House at Harkness Memorial State Park in Waterford CT I assumed it was used to turn horse drawn carriages completely forgetting that the estate was built after the turn of the century. Then I saw something that referred to it as a firing pivot - which totally confused me and led me to this post. First patented by the Canton Foundry & Machine Co in 1910 under patent number US 971796, it was actually used to turn automobiles. Imagine that?
To view the complete patent go to the US Patent website here , search for the number & then click "images".
Amylynne (Baker) Murphy