In this photo, guard Derrel Gofourth (57), linebacker Rich Wingo (50) and center/future color commentator Larry McCarren (54) take the field:
Check out the right angle on those 5s, and contrast with the modern hooked numbers:
The Packers' hooked 5 goes all the way back to the Lombardi era:
Perhaps this straightened digit indicates a "house font" from their uniform supplier at the time. This was far more common in the 1960s, but doesn't seem all that unlikely in 1979.