Well, I can't help with the serial number.
Assuming it has the original barrel...the chart in the back of the Blue Book says "Code located on barrel, left side of frame"
Month of Manufacture
BLACKPOWDERX, with B=Jan., L=Feb., etc.
Year of Manufacture
1950 = WW
1951 = XX
1952 = YY
1953 = ZZ
1954 = A
1955 = B
1956 = C
1957 = D
1958 = E
I just checked the 870 Exp. Mag. I bought for the fall duck season in 1993 and it has an EN on the barrel.
E = Oct., N = 1993. FWIW, I pulled the receipt on it - 11/16/93.
The chart starts with 1921 (= M) and ends with 1999 (= T).
They repeated the alphabet as necessary, leaving out a lot of the I's, O's, Q's & V's, and used double letters from 1943 to 1953.
1976 through 1979 used 1, 0, Q & V.
Starting in 1980, with an A, it looks like they started using the whole alphabet.
Hope this helps.
John