AFAIK, The receiver cannot be dated by SN, but the bbl can, AND hopefully the bbl is original to the receiver.
The Remington Date Codes are always stamped into the barrel's lower side, near the rear - just ahead of the receiver.
The first letter of the Code is the month of manufacture, followed by one or two letters which are the year of manufacture.
As noted above, for shotguns with removeable barrels, the code will be valid for the manufacture of the barrel; maybe for the receiver, as barrels do get switched around.
According to the "Blue Book of Gun Values", the coding continues as follows (edited to reflect the intro year of the OP's Model 870) :
Month Codes: [first letter]
B - L - A - C - K - P - O - W - D - E - R - X
1 - 2 - 3 - 4 - 5 - 6 - 7 - 8 - 9 - 10 - 11 - 12
Year:______Code: [second (and third * letters]
1950____*___ WW
1951____*___ XX
1952____*___ YY
1953____*___ ZZ
1954_______ A
1955_______ B
1956_______ C
1957_______ D
1958_______ E
1959_______ F
1960_______ G
1961_______ H
1962_______ J
1963_______ K
1964_______ L
1965_______ M
1966_______ N
1967_______ P
1968_______ R
1969_______ S
1970_______ T
1971_______ U
1972_______ W
1973_______ X
1974_______ Y
1975_______ Z
1976_______ I
1977_______ O
1978_______ Q
1979_______ V
1980_______ A
1981_______ B
1982_______ C
1983_______ D
1984_______ E
1985_______ F
1986_______ G
1987_______ H
1988_______ I
1989_______ J
1990_______ K
1991_______ L
1992_______ M
1993_______ N
1994_______ O
1995_______ P
1996_______ Q
1997_______ R
1998_______ S
1999_______ T
2000_______ U
2001_______ W
2002_______ X
As maybe seen, the year code letters duplicate; some knowledge of when the model was introduced should resolve the actual year of manufacture.
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