Nice. Got to love that thrashed furniture. Boba Fett cool.
A serial number starting in "W" points to a production date between 1984 and 1990. Seems to match the stamp on the barrel, but who knows?
I think the mix & match theory is accurate. Luckily, this is an 870, and parts are pretty freely interchangeable. Even more lucky, you ended up with two out of the three components required for the flex-tab update. You need all three to have the full fix that prevents a feed malfunction from locking up the action (more info on that
here), but it sounds like you already have the expensive one (the breech bolt) and the PITA-to-install one (the carrier assembly), so all you need is a new slide assembly to have a fully-updated action. Slide assemblies are only something like $17 - $25 if you shell out for a brand new one, and they literally drop right in. Definitely a recommended upgrade for a defensive gun.