I did a little research, and found out that the small plastic piece that didn’t get back into the laptop is a holder for the AirPort antenna if it is not connected to an AirPort card. Since I do have my AirPort card installed, and the antenna connected, the piece isn’t strictly necessary. But now that I know what it is, I may as well put it back in for completeness’ sake.

Here is the explanatory diagram:

Thanks to ***mgilliland|http://www.broadbandreports.com/profile/439458*** at ***Broadbandreports.com|http://www.broadbandreports.com/*** for finding this info, and also for originally recommending ***Pre-Owned Electronics|http://www.preowned.com/*** as the source for my replacement part!