Best I can remember & checking the 1978 was the only year for that shaped skeg. The part numbers indicate that, but other things can cause different part numbers. The 1977 was different internally (front bearing assy.) The 1979 should have been the same on the inside, but I don't remember the curve in the leading edge of the skeg?
The reason I remember this motor so well is because it is my favorite V-4 cross flow motor. The last year for the small bore 92 cu in block, but the first year for the new TNT between the stern brackets. It wasn't the most powerful V-4, but was the most bulletproof! They just didn't give problem with the powerhead. When you see one today most are still running the original powerhead. I hope you have one.