My first retractor problem

Just having fun for the first time with retractors:

White might have checkmated on the last move.

White to take back his last move, and mate at once:

The only time I've come across retractors before was in obstetrics!

I'm new to this type of retrograde analysis but so I don't spoil things for anyone else, I'll post what I think may be the answer in white type below:

The only way I can see a mate in one move is to place a white knight on f8. That means the rook on that square has just been promoted from a pawn, so 'unpromote' from the rook to a knight?

That's the right answer!

I went to the Wiki' page for this to find out exactly what they are & there's a good problem there..

http://en.wikipedia.org/wiki/Retrograde_analysis#Example

But was the pawn on e7 or f7 before promotion? Think this one might be cooked unfortunately!