Thanks for the link to the early history of Gradall truck-mounted excavators.
As to the world’s first hydraulic excavator… many would say that that these early truck-mounted machines were just attachments, no-one produced a complete hydraulic excavator until the Italians produced a prototype wheeled hydraulic excavator in 1948 (which was latter to become the Yumbo brand).
The French always claim that the world’s first hydraulic excavator was the 1951 Poclain TU, but this was a tractor-pulled and driven machine, not a stand-alone excavator.
Discounting the early hydraulic machinery that used water, not oil, probably the world’s first hydraulic excavator was the 1897 Kilgore (built in Minneapolis).