Help identifying 1979 100hp Evinrude

[grrrrrant]

I believe it is one of the 3 following model numbers:

100990S
100993S
100999S

Not sure which it is. The ID plate is scratched to hell (doesn't look intentional)

Does anyone know what the difference in these 3 models are? It may help me determine which model I have.

Thanks in advanced!