So... I finally got around to putting a Timney trigger in the aforementioned 77 Mk II in 7x57mm.
Removing that front action screw had me more worried than I ever have been with any rifle that wasn't already a beater, or something that came to be as a basket case.
It was not moving.
I was right on the edge of the maximum amount of torque I was willing to apply to that screw, when something finally snapped.
I wiggled the screwdriver a bit, to see if the head had sheared. No.
So, I went back to attempting to remove it.
It was stiff, and felt cross-threaded, but was coming out.
End result: Someone had dipped the entire threaded portion of the screw in red loctite.