mercury 8 9.9 4-stroke powerheads

[Lewis]

As I understand it, Mercury (and Nissan and Tohatsu) used the same powerhead for the 8HP and 9.9 (or 9.8)HP 4-strokes from at least 2005-2015. But I've also seen information that at some point, the 9.9 and the 15 4-strokes used the same powerhead.
Can somebody please shed some light on this?
Thanks!
Lewis

 

[Lewis]

Just to close the loop at least a little, a powerhead from a 2005 Merc drops right in to a 2013 outboard. I can't say for sure about the Nissan and Tohatsu powerheads, but I'm pretty sure that they would work as well.
And I've got to say, I'm pretty excited to finally have an outboard made in this millennium!