Bearing size is the difference.
Got these part #s from Rock Auto.
1978
NATIONAL Part # A2 {Taper Bearing Set; Cone Length=0.475" Bore=0.75" Outer Diameter=1.781" Width=0.61 StoundOut=0.135" Cup Length=0.475" Cone Radius=1.046"} - Outer
NATIONAL Part # A6 {Taper Bearing Set; Cone Length=0.465" Bore=1.25" Outer Diameter=2.328" Width=0.625 StoundOut=0.16" Cup Length=0.465"} - Inner
1979+
NATIONAL Part # A3 {Taper Bearing Set; Cone Length=0.55" Bore=0.8437" Outer Diameter=1.9687" Width=0.69 StoundOut=0.14" Cup Length=0.55" Cone Radius=1.046"} - Outer
NATIONAL Part # A6 {Taper Bearing Set; Cone Length=0.465" Bore=1.25" Outer Diameter=2.328" Width=0.625 StoundOut=0.16" Cup Length=0.465"} - Inner