AND, it is in the negative, so I can't imagine this construction in the affirmative. AND, I can't imagine the other (using "a no ser por..."), and they did not give an example of that.......
In the affirmative it would be:
De haber sido por ti, no habríamos ganado el premio.
In many cases when the first sentence is negative the second is affirmative and vice-versa.
De no haber sido por ti, habríamos ganado el premio.
However, you can say:
De no haber sido por ti, no habríamos ganado el premio.
"A no ser". I think the only way I use it is when I say
"a no ser que......"
It means "if we had done what you said..." or something like that.
