Give H&R a call, they didn't use O or Q, otherwise it would corrupt the rest of the sequence which as you can see isn't missing any years, just those two second letters.
Are you sure it's not something older that is N
0(zero)12345 which would make it a 1953. Is it marked Pardner just on the barrel and is the underlug marked with the same last 3 digits of the frame serial number? Their toll free number is (866)776-9292.
Tim