The stamped date is the date the stocks started into the finishing process(pages39-41). The serial #'s were written on the forearm and buttstock in order to keep them together, as they were individually hand fitted.
Now how rare is it? You did not tell us which configuration it is and there are 10 possibles here. Most likely it is a BB(later cataloged as a 1B). "BB" is etched on the right side of the forearm hanger(page 49). About as many BB's were made as the total of the other 9 configurations. Check out Rarity estimates on my Website and let us know which configuration you have.
