Ben,
A serial number of 51 would definitely be of interest and of increased value. The serial number is located on the bottom of the receiver just below the wooden fore end. As in the photo attached, I hope. From your photo it does not look like there is any blue left on the receiver and it may have been buffed or sanded in the past to the point that the serial number is gone. I have seen that happen numerous times.
There should also be a caliber marking on the left side of the barrel between the rear sight and the receiver. Sometimes there are small numbers very near and in front of the trigger that are factory inspector numbers stamped on the rifle. Please post some more photos to clear up these questions.
2bit