Hi, Bruce - I think maybe McGrath's message partly answers your question - I get the impression that some of the music isn't displayed for two reasons:
1. The project isn't quite done.
2. For copyright reasons, music newer than 75 years old isn't being displayed (apparently, if the song is more than 75 years old, even though the printed publication isn't, they'll display it - some of the displayed music has photos of performers still alive)

Looks to me like maybe nothing in the box you're interested in has been put on display yet. apparently, though, they're willing to mail copies for 25 cents a page. As shown in McGrath's message, their e-mail is joan@musicbox.mse.jhu.edu
-Joe Offer-

For those who might be interested, making a clickable e-mail address is almost like making a clickable web link, but you have to add mailto:
To make my address clickable, I'd just substitute angle brackets for square ones in this:
[a href=mailto:Joe-Offer@msn.com]-Joe Offer-[/a]