I observed that the K3 driver and the XXL driver were similar in size, around 26-27 mm (i e smaller than the DKK32 driver). Different diaphragms though, as you point out.
Pics of XXL drivers courtesy akgfan, different diaphragm pattern and closed back (except the "bass holes"):
Same XXL driver in the K242 HD:
And here are some pics of the 1984 K3 and driver, basically open back except for the ubiquitous white paper filter:
Edited by MDR30 - 3/25/11 at 9:58am