My initial thought was that this bastion was refurbished in the 1870's, during a period in which many US fort improvement projects started but were never finished, due to lack of both funds and interest in post-Civil War America...but Fort Clinch was placed into Caretaker Status from 1869 to 1898, and the one guy who would've been there to keep an eye on things would not have strenghtened this bastion and built this mount by himself.

The nice folks at Fort Clinch State Park inform me that this mount, as well as five others found along the fort's walls, were built for 15" guns, the largest of the fort's armament. Were these 15" guns ever affixed to these mounts? An excellent question, which apparently only the period-dressed guides in the fort itself can answer, and they're in 1864, so they don't have phones on them.