Vic,
The correct name is not Peter Longo but rather Peterlongo. Johann Peterlongo opened a big gunshop in Innsbruck in the 1850s and the marque existed long after his death. A Google search for 9x71 Peterlongo will give you drawings with principal dimensions, etc. I do not have any experience with this obsolete cartridge but am quite sure that the pertinent rifle must be a nice Austrian Mauser.

With kind regards,
Jani