When did the aluminium keyboards' scissors change

Frankly, does it matter?

Implied in Dave's post was "is there a reason you need to know?" Is it simple curiousity or are you trying to replace the keycaps on a keyboard, for example? Curiosity is not a bad thing, but you could have simply stated so without repsonding with a sarcastic remark. (And yes, Dave's remark is sarcastic, although less so. But you could have deflected the sarcasm with something like, "I just wanted to know.")

That said, I do remember when the iPad1 first came out, there were reports of early Apple keyboards not working and that later keyboards had a different firmware that did work with the iPads. If that is true, then it would seem reasonable to conclude that there were multiple models of the Apple keyboard. (I think they even have different model numbers.) If those pictures you've posted were from keyboards in your posession, then you should be able to confirm that they do in fact have different model numbers.

That then brings us back to the question of why does it matter to you? The only instance I can think of is if you're trying to replace the keycaps with keycaps from another keyboard. (Which if you stated so would have also deflected the sarcasm.) In that case, you may be out of luck if the keycaps aren't interchangable. Same would be true of keycaps from any other keyboard. You'd need ones from the same model keyboard.

And if that leads to the next question of "why would Apple change the switches?" then the answer is simple: it happens all the time in manufacturing. Sometimes you have to change suppliers because of a price difference or because the supplier can't supply enough parts. (i.e.: Thailand flood, Japan earthquake.) Regardless, they're different, but they do not affect how the keyboard works for everyone else, so the point is moot unless you have a technical reason like keycap replacement.