I have to wonder why such tracking tags would have been left in a backbox / cabinet but removed from the pf?

So my guess is it was for the final QC / playtest / audit check before the game was packed and shipped. The game has a S/N tag with a checklist and/or a place to sign off on inspection before shipment: Last guy on the line is assigned to perform the checks, rip off the tag, sign it, and file it. PF is probably raised during this check-off process so it's a good place to keep the tag until just before it's lowered the last time before the glass and lockdown are installed pre-shipment.

Think about it: the checklist tag left on the game, which we all see, is almost kinda meaningless: "Oh yay, somebody put various mark(s) at various places in the list (but not all them, ever notice that?) and the game still doesn't work. Whoo, some checklist, what do I do now?". Well, you call Williams, give them your S/N, they look it up, and hopefully find the teartag to confirm it passed inspection, who actually signed/certified the tear tag, and hopefully have an internal review. (This would also help ensure preprod/test/sample and/or factory-damaged/lost games aren't out for liability in the wild).

And perhaps it was even worded so that if you (probably only a distributor in those days?) - did get a game with that tear-off still attached, you call WMS, at which point they probably redeem the tag for some kind of apology refund or service credit or something. Point being in any case, incentive was there for these not to survive in the field.

That's the process I imagine, at least.

Imagine if Stern did that, haha....