Thanks for the clue Large. Here are a couple of pics:
http://picasaweb.google.com/toddmdavenport/UntitledAlbum/photo#5214854286487036130
http://picasaweb.google.com/toddmdavenport/UntitledAlbum/photo#5214854283837331266
the plug fits neatly into the hole with a little resistance. It actually stays in well enough to hold my pannier, but I do not trust it with my kid.