That's not a cheap old gun - it was a medium-grade gun when it was made, and nicer than most guns you'll find coming out today. The scalloped boxlock design, sideclips, design of the fences (I'm not going to call them clamshells, but they're similar) and engraving point to it being a medium-grade gun. I'll bet (can't tell from the photos) it has a hidden third bite - another indicator of quality.
More likely the pitting was the fruit of the corrosive primers in use the first 20 or 30 years of this gun's service, though the rock salt does sound better.
Your grandfather did restock it - the buttstock would have had a checkering pattern consonant with that on the fore-end and your photos don't show any. Moreover, the grip looks too thick to be original - French and Belgian guns of that vintage seem pretty uniformly to have very slim, graceful grips be they PoW or, as yours, English. That, and it appears the fore-end has some figure in it, which the buttstock does not. Euro makers were pretty uniform in having the amount of figure between fore-end and butt be consistent, and your gun isn't.
And, to restock a scalloped boxlock is no small "handyman" project. Your grandfather did pretty well after buying very nice.
