The date stamp stamped in the aluminum states it was manufactured 3/80. March / 1980.
So that might narrow it down a bit.
However the date stamp isn't 100% conclusive as it just means the date of the aluminum piece.
Any engineering numbers on the parts themselves? ; etc... That could also narrow it down to a year or two.
If I was to hazard a guess, with the date stamp . . . I'd say 1980/1983. but wouldn't guarantee it.
Truck: 1981 F-150 Explorer / Engine: 300-6 California MCU Feedback System / Trans: T-18 - 4 speed / 2.75 Ford Rear Axle Open Diff.