I lost the original eyecup piece on my D40 last year because it just fell off by itself. I put it back on a few times but the last time it fell off without my noticing it. The manual states that the original eyepiece is the DK-16, but I didn't want to get another one that would have the same problem.
The DK-20 is a great replacement eyecup for the D40 because it slides onto the lip found around the viewfinder and locks into place. I am confident that I won't be losing this piece because it fits so well and it is so secure. If you need a replacement for your D40/D40x I recommend it!