I included DropPrint•USB in the Print66 package mainly to help you find the model and printer names for USB printers attached to your Mac. You could also use it for testing purposes : if Print66 doesn't achieve to print and DropPrint achieves, then I have done something wrong...
From the Original ReadMe file (included in Mac_OS_DDK_v1.41)
DropPrint•USB demonstrates a method for identifying attached USB printers which are supported by the Apple USBPrinterClass driver and to send data to these printers. This sample program works with printers supported by the Apple USBPrinterDriver. One requirement is that the printer must respond to the "1284 Get Capability String" request. The Apple USBPrinterDriver uses the response to this request to register an entry in the Name Registry. This sample searches for these strings to display the names of the available printers.
DropPrint•USB is based on the DropShell 2.0 sample code, originally written in 1994 by Leonard Rosenthol, Marshall Clow, and Stephan Somogyi. The modifications and USB specific code were implemented by Olav Andrade.