HI scoutjim,
I'm also thinking acid-treatment might have caused this effect. The pitting seems too uniform to be a struck-through error.
If I were willing to sacrifice a Walker, I might drop one into some dilute nitric acid and observe the effects. If the coin sits at the bottom of a beaker with the obverse side up, it will corrode more rapidly on that side. The result might look a lot like this coin.
If it's an acid job the edge reeding would probably be eroded and the weight would be low. I weighed some circulated Walkers grading from Good to XF and none weighed less than 12.0 grams; 12.5 grams is standard weight for a new silver half. If your coin weighs less than 12.0 grams I'd be pretty sure that metal has been chemically removed.