Okay, now I'm reasonably sure that 37 is the correct part. It is definitely not 38, because 38 is clearly illustrated as being a trim ring, not a cover plate.

The reason it's called an "assembly" is that it includes some little metal spring clips, which I had to remove in order to get it to work for my purposes. I also had to trim some bits of the plastic carefully so that it would seat properly.

The original thread where I discovered the part and reported on it is here. However, the link that I posted in that old thread is broken because Majestic Honda changed their web page catalog code.