One of my buddy's has his Dad's 870 Trap Gun and it has the usual serial number on the receiver but also has the same, matching number on the barrel.
The barrel code is a bit "challenging", it has three letters, EBY (or BBY, the first letter may be incompletely stamped); this doesn't fit with any code I am familiar with.
The stock have the small, diamond pattern cut checkering that you often see on the 1950's guns.
The barrel extension has that distinctive "ring" wher the barrel transitions from the extension, just ahead of thefront of the receiver that you also see on the earlier guns.
His Dad passed away, in his 80's, a few years ago, since it was Fathers Day, he brought it out to the club; ran a 24 and 25 with it at 16 yards.