3 letter barrel code

[sreming]

I just picked up an 870 16 gauge. The barrel code is 3 letters.
EEW is on the barrel. The receiver serial number is 6 numbers then W. No prefix in from of receiver serial#. The rear
Stock is plain. No checkering with a hard plastic butt plate.
The front stock has slits cut vertically
E - Oct
E - I believe is 1958.
W - what is this third letter for. It doesn't match any of the year codes that I find in listings?

[BKinzey]

There are a couple of charts but I think they are more like guesstimations rather than definitive.

https://oldguns.net/sn_php/remdates.php

[hauntingselection]

I just picked up an 870 16 gauge. The barrel code is 3 letters.
EEW is on the barrel. The receiver serial number is 6 numbers then W. No prefix in from of receiver serial#. The rear
Stock is plain. No checkering with a hard plastic butt plate.
The front stock has slits cut vertically
E - Oct
E - I believe is 1958.
W - what is this third letter for. It doesn't match any of the year codes that I find in listings?

Thanks! I came across that chart too, but the third letter “W” is what’s throwing me off. I haven’t found a clear explanation for it yet.