I am looking at a head unit on eBay and it is an 2005-2006, I can tell by the *** in the photo. What I want to know is will it work with my H/K system that I have in my '04.

Here is the part number #6 921 965 and it's made by Alpine

Thanks for the help!

 

I was under the impression that all parts of the H/K system were unique and not interchangeable with other stuff.

 

HU are all the same. The later HU has a knurled ****, and is quite a bit bigger than the pencil eraser of the earlier model. Still tough to see in pics, though.
Either one will work with your '04 HK.
(I switched out my '04 for an '06)