You can "date" it by checking the ser. # at this website.. Remington Dates of Manufacture.

At a family funeral last year, an older cousin who I rarely see offered me his and my uncle's (Deceased) hunting guns. There wasn't anything rare or valuable but there was a Rem 870 20 gauge, much like the one you describe. It was made in 51. About a week or so after he gave me the gun, the manual arrived in the mail! The neat thing about the older 870s is that they were all made on the same reciever...all the modern accesories for the 12 ga will fit my gun. I've been toying with the idea of setting it up for 3-gun matches.

Edit: Ooops...I see that you already have the date correct....maybe that site may be of interest to others. My gun ends in "XX" = 1951