Is generally found on the frame as the lowercase
"i" shown in the photo illustration. This location may vary slightly for each Luger encountered as the stamp was undoubtedly struck by hand, and location was not that critical.
Your
"o" should be found in approximately the same location, and sometimes, but not always, the letter is also found on the underside of the barrel under the serial number stamp on the barrel of an otherwise unmodified Luger...
regards,
John Sabato
http://boards.rennlist.com/upload/suffixlocation.jpg