There are earlier keyboard models than the KK3. The KK3 wasn't called a Kindle Keyboard at launch
The
2nd Keyboard model
The KK3 came out between DX and DXG. So in one sense the 4th model with a keyboard.
The DXG is the last model without an upgrade to KF8/azw3.
The KK3 with last FW does indeed to KF8/azw3 well, but on original FW it didn't support it. I don't know which update added the azw3/KF8 and extra fonts. Allegedly the later DXG model is basically a DX with a newer panel and not enough RAM for the KF8/azw format rendering.