Replacing subpanel that was fed with 3 wires
Your feeder appears to be in metal conduit, which is the grounding conductor
Since your feeder appears to have been run as wires in a metal conduit, that conduit would serve as your grounding conductor provided it is continuous back to the main panel.  I would turn off the feeder at the main panel and do a continuity check from the neutral lug on the panel to the panel cabinet, though, to make sure of this.
... and that &quot;hook&quot; wasn't ever needed in your situation
Note that the &quot;hook&quot; you describe, which is more properly called a lug on a bonding bushing that was present in the old installation, wasn't actually ever needed in this case, since the conduit does not have service entrance wires in it, just feeder wires, and your circuits are running at less than 250V to ground.  (Bonding around concentric/eccentric/impaired KOs in the fashion you describe is only required for service entrances and circuits upwards of 250V to ground, as per 250.92(B) and 250.97 respectively.)
You will need to have those open knockout holes plugged though
Unless the contractor is planning on using those open holes in the top of the panel for a cable run when they come back, they'll need to put knockout plugs in the open KOs in the top of the subpanel in order to keep junk from getting into the panel and causing trouble there.  (They're a Code violation for that reason.)