There are five files in the folder. The one that contains the name information is the NBImageInfo.plist file. Open this file in BBEdit or another editor that maintains line breaks. You could even use Apple's Property List Editor if you wanted.

Find the line that looks like this:

<key>Name</key>

The next line is the string containing the image name that is displayed in Startup Disk:

<string>LIBRn310(Mercury)p</string>

Change the string between the <string> and </string>, then quit and save the file.

Sometimes it takes stopping and restarting the NetBoot/DHCP process to get the change to be re-read. You can stop and restart the service through Server Settings in Jaguar server or Server Admin in Panther server.