From the FAQ:
The right covers differ in mounting at the top forward mount position where the mount tab is rotated 90 degrees. The 390 and 404 covers swap among frames F style and late K model, but not with the earlier K model 323 frames/covers.
What doesn't show in the pics, possibly, is that the 323 covers are significantly fatter/deeper than the 390 and 404 styles. About an inch I guess.
Ok, so how do I know what one I have? The 323, 390 or the 404?